Category: Access

  • Microsoft Access Performance Tips to Speed up Your Access Databases

    Over 100 Tips for Faster Microsoft Access Databases

    This paper outlines specific performance tips you can use to improve the speed of your Microsoft Access applications. These tips apply to all versions of Access including Access 2016, 2013, 2010, 2007, 2003, 2002, 2000, and 97.

    Some of the tips may contradict each other but they are offered because bottlenecks differ in each situation. Some tips may make things run faster on your system, while others degrade performance. You should evaluate each tip as it applies to your specific application running on your specific hardware.

    Good luck!

    Microsoft Access Table Performance Tips

    Normalize Your Tables

    Normalize your data so the same data is stored only in one place. Remember that time is also a dimension and you may need to store historic data as well. Read our paper on Data Normalization Fundamentals for more detail.


    Every Table Should Have a Primary Key

    Every table in your database should have a primary key. This allows your database application to quickly locate specific records. Additionally, you cannot create secondary indexes on a table’s fields unless that table has a Primary Key.


    Primary Key Should be One Field and Numeric

    The primary key should only be one field and ideally numeric and meaningless. Primary keys define the uniqueness of each record which can be accomplished efficiently with a single number. The easiest is using an AutoNumber field in Access or Identity column in SQL Server. The primary key is also used in each secondary index for that table, so the smaller the better. Multi-field primary keys and non-numeric text fields are less desirable.

    That said, some tables should simply use text fields as primary keys because they don’t change much and the tables are relatively small. For instance, a list of countries or states (no need to create a separate number for each country or state).

    Having a meaningless primary key means the index is stable even when data changes. Otherwise, changes in the primary key have a ripple effect through each secondary index and any other tables bound by referential integrity.


    Tables Should Participate in Relationships

    Related tables with one-to-one or one-to-many relationships should implement referential integrity with cascading deletes and/or updates to ensure orphan records are not created. With cascading deletes, the removal of the parent record, automatically deletes the child records in the corresponding table. Access (the Jet Engine) automatically takes care of this for you without the need to write any code, which is great for maintaining data integrity.

    With a relationship established, a hidden index is created for the foreign key, so links between those tables are faster in queries. The link is also automatically drawn for you when you put the two tables on your query designer.


    Eliminate Unnecessary Subdatasheets

    By default, Access creates subdatasheet views between related tables. This is nice if you want it, but is a huge performance hit every time you open the table. Set this property to None if you don’t want it. Here’s more information on the Subdatasheet Name Property.


    Choose the Optimal Data Types

    Choose the best data types for your fields. By choosing the optimal data type, you can decrease both the disk space used to store data, and the time it takes Access to retrieve, manipulate, and write data. The general guideline is to choose the smallest data type possible to store a particular type of data.


    Add Secondary Indexes As Necessary

    If your tables are large and you search on a field or use it in a join, create a secondary index on the field(s). Secondary Indexes offer performance gains on an order of magnitude.


    Don’t Over-Index

    Just as it is important to add indexes to fields that need it, it is important to avoid indexing fields that don’t need it. Every index adds to the time it takes the database engine to update, delete and add records.


    Don’t Index Fields with Lots of Identical Data

    Don’t apply indexes to fields that contain much the same data. For example, indexing a Yes/No field is almost always a performance degrading operation. Similarly, if you have a number field that only contains two or three values, an index wouldn’t be a good idea. To check the number of unique entries in an index, use the Access DistinctCount property. Compare this value to the number of records in the table and you can quickly see if the index is doing you any good.


    Keep Indexes As Small As Possible

    When creating a multi-field index, index only as many fields as are absolutely necessary.

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    Linked Databases and Linked Tables

    Keep an Open Handle to Every Linked Database

    You can significantly increase the performance of opening tables, forms, and reports that are based on linked tables by having Microsoft Access keep the linked table’s database open. To do this, open a Database variable in Visual Basic code using the OpenDatabase method. Keep this variable open as long as your application is running. This forces Access to keep that database open, making access to linked tables much faster.

    For more information, read our tip on Increasing the Performance of Linked Databases


    Minimize Record Navigation

    Minimize record navigation wherever possible on linked tables, especially large SQL Server tables. Only use the Page Up and Page Down movements, and the Move last movements when absolutely necessary.


    Use DataEntry Mode

    If you are only going to be adding records, use the DataEntry command on the Records menu. This data access method is more efficient for adding records because existing records are not read from the database.


    Create a Data Entry Form

    If you often have the need to add new records to a linked table, consider creating an “Add Records” form and set that form’s DataEntry property to Yes. This prevents Access from attempting to retrieve all the existing records in a linked table when you need is to enter new records.


    Limit the Data Returned by Queries

    Limit the number of fields and records returned by using filters or queries. This reduces the amount of data that needs to be read from the linked table, thereby according faster performance.


    Don’t Use Domain Aggregate Functions

    If a query is based on one or more linked tables, avoid using functions (built-in or user-defined), or domain aggregate functions in the query criteria. When you use domain aggregate functions (such as DLookup), Access must fetch all records in the function’s data source to execute the query.


    Release Locks ASAP

    To improve multi-user concurrency, assume that other users will be trying to edit data in the same linked tables that you are using. In other words, keep records locked only as long as is necessary.

    SQL Server and ODBC Performance Tips

    Link ODBC Tables

    If you are going to be accessing a SQL database table, link the table permanently. This makes opening that table much faster in subsequent attempts. This is because linked tables cache a lot of information about the source table in your database, making the retrieval of the same structural information unnecessary after the linked table is created.


    Minimize Server Traffic

    Minimize the amount of data returned from the server. Do this by structuring your queries to return only the fields and records needed. Reduce the number of fields returned, and put constraints on the query by using the WHERE clause.


    Use Snapshots When Appropriate

    Don’t open Dynaset type recordset object on SQL database tables unless you need to add or edit records, or need to see the changes made by other users. Instead, consider using Snapshot recordsets which can be faster to scroll through. Of course, Snapshot recordsets can take longer to open since they require a full read of the source data.


    Use Dynasets for Large Record Sets

    If you need to retrieve a large number of records, use a Dynaset instead of a Snapshot. Snapshot type recordsets must load all records from the data source before becoming available, whereas Dynasets are available as soon as the first 20 or so records are retrieved. Also, when using a Snapshot against large ODBC data sources, you run the risk of running out of disk space on your local computer. This is because all data is downloaded into RAM until the amount of RAM is exhausted. Then, the database engine creates a temporary database to store the contents of the snapshot. In a nutshell, when you open a snapshot, you need at least as much disk space as the largest table you are opening.


    Take Advantage of the Cache

    Use cache memory wherever possible when using external SQL data sources. Microsoft Access forms and reports have automatic caching mechanisms. When using recordsets in your Visual Basic code, use the CacheStart, CacheEnd and FillCache methods to maximize cache effectiveness.


    Don’t Force Local Query Processing

    Don’t use query constructs that cause processing to be done by Access on the local computer. The following query operations force the Jet database engine to perform local data processing:

    • Join operations between table that are linked to different data source (I.e. a join between a SQL table and a linked Access table)
    • Join operations based on query that uses the DISTINCT keyword, or a query that contains a GROUP BY clause.
    • Outer joins that contain syntax that is not directly supported by the remote database server.
    • The LIKE operator used with Text or Memo fields
    • Multi-level grouping and totaling operations
    • GROUP BY clauses that are based on a query with the DISTINCT keyword, or the GROUP BY clause.
    • Crosstab queries that have more than one aggregate, or that have field, row, or column headings that contain aggregates, or that have an ORDER by clause
    • User-defined functions, or functions that are not supported by the remote server
    • Complex combinations of INNER JOIN, LEFT JOIN, or RIGHT JOIN operations in nested queries.

    Use FailOnError For Bulk Updates

    If you are using bulk update queries, optimize performance on the remote server by setting the FailOnError property of the Querydef object, or query to Yes.


    Use ODBCDirect

    ODBCDirect gives you almost direct access to server data through ODBC. In many cases, it is a faster and more flexible way to hit server data than that traditional Jet/Linked table technique.

    Microsoft Access Database Performance Tips

    Split Your Database into Two Databases

    You can improve performance and application maintainability by splitting your Access database into two databases. The “application” part of your database holds all objects except tables, and is linked to the “data” part that contains the actual tables. For more information, read our technical paper Splitting Microsoft Access Databases to Improve Performance and Simplify Maintainability


    Use a Current Workgroup Information File

    If you are using a workgroup information file (SYSTEM.MDA) created with a previous version of MS Access, convert it to the current version of Access for optimum performance.


    Use the Access Performance Analyzer

    Microsoft Access has a useful performance tool built right into the product. From the Tools menu, select Analyze, Performance. The Performance Analyzer allows you to select all objects or specific objects, and then runs an analysis looking for potential problems. The Performance Analyzer does not find all of the items that Total Access Analyzer does, but it does offer some tips.


    Run the Microsoft Access Table Analyzer

    The Access Table Analyzer makes it easy to properly normalize the data in your tables by breaking tables with repeated or improperly structured data into two or more tables. This tool is available from the Access Tools, Analyze menu


    Reduce the Size of Graphics in your Microsoft Access 2007 or Later Databases

    If you embed graphics on your forms or reports, Access 2007 (or later) can store them much more efficiently. Access 2007 can convert graphics into much smaller PNG formats to significantly reduce the size of your databases. This does not affect graphics already on your forms and reports but helps if you add new graphics or replace existing ones. To activate this feature, change an Access setting. From the Access Office button, choose Access Options, Current Database. At the bottom of the Application Options section, set the Picture Property Storage Format to: Preserve source image format (smaller file size)


    Compact Your Database Often To Reclaim Space

    Compacting your database reclaims unused space and makes almost all operations faster. You should do this on a regular basis. Also, be sure to compact anytime you import or delete objects in your database, or compile and save VBA code.

    Learn more about Total Visual Agent for a system administrative tool to schedule compact and backup your Access databases on a regular schedule.


    Make It Look Faster

    If you have exhausted all other performance optimization techniques, consider making your application “look” faster. Do this by displaying status messages and progress meters as your application loads forms, runs queries, and performs any other operation that may take a bit of time. While this doesn’t make your application run faster, it appears to run faster.

    Microsoft Access Query Optimization

    Compact Often to Update Statistics

    Compact the database often. When you compact the database, you reorganize records so that they are stored in adjacent spaces, making retrieval faster. Additionally, compacting a database updates its data statistics, which can be used by a query to run faster. You can force a query to recompile (which in turn causes it to use the latest statistics) by opening it in design view, saving it, and then running it.

    You may want to defragment your disk using a program such as the Disk Defragmenter that is part of Windows before compacting your database. This leaves contiguous free disk space immediately after the database file. In theory, this make future additions to the database occur faster. You may want to experiment with this on your system.


    Index Query Criteria Fields and Join Fields

    Index any fields in the query that are used to set criteria. Index the fields on both sides of a join. Alternatively, you can create a relationship between joined fields, in which case an index is automatically created.

    Search Access Help for: Index


    Use Identical or Compatible Datatype In Join Fields

    Fields that are joined in query should have the same data type, or compatible data types. For example, the Long Integer DataType is compatible with the AutoNumber data type.


    Limit Fields Returned by a Query

    Where possible, limit the number of fields returned by a query. This results in faster performance and reduced resource usage.


    Avoid Calculated Fields and IIF Functions

    Avoid calculated fields, or fields that use expressions in subqueries. Pay special care to avoid the use of immediate If (IIF) functions in sub-queries.


    Don’t Use Non-Indexed Fields for Criteria

    Avoid using non-indexed fields or calculated fields for criteria restrictions.


    Index Sort Fields

    Index any fields you use for sorting. Be careful not to over-index.


    Use Temporary Tables to Eliminate Running the Same Queries Over and Over

    If you are processing data that’s used multiple times (for instance on multiple reports), it may be faster to store intermediate results in temporary tables rather than running a series of Select queries many times. Create a temporary table to store your results. Empty the table and fill it with your data using and Append query. You can then use that table for multiple reports and forms.


    Avoid Domain Aggregate Functions on Foreign Tables

    Do not use domain aggregate functions (DLookup for example) in queries to access data from a table that is not in the query. Link to the table and set criteria accordingly, or create a separate aggregate (totals) query.


    Use Fixed Column Headings in Crosstab Queries

    Wherever possible, use fixed column headings in your crosstab queries with the PIVOT syntax.


    Use BETWEEN Rather than >= and <=

    Between lets the search engine look for values in one evaluation rather than two.


    Use Count (*) To Count Records

    If you use the Count function to calculate the number of records returned by a query, use the syntax Count(*) instead of Count([fieldname]). The Count(*) form is faster because it doesn’t have to check for Null values in the specified field and won’t skip records that are null.


    Compile Each Query Before Delivering Your Application

    When you compact your database, its data statistics are updated. When you then run a query, these updated statistics are compiled in the query’s execution plan. This sequence of events results in the fastest possible query. Before you deliver your application, compact the database, and then force each query to be recompiled. You can force a query to recompile (which in turn causes it to use the latest statistics) by opening it in design view, saving it, and then running it.


    Take Advantage of Rushmore Query Optimization

    Microsoft Jet uses Rushmore query optimization whenever possible. Rushmore is applied to queries run on native Access data, and on linked FoxPro and dBASE tables. Other linked data types do not support Rushmore optimization. To ensure that Rushmore optimizations are used, create indexes on all fields that are used to restrict a query’s output. If you have queries that don’t contain indexes on fields used in the query’s restriction clause, Rushmore is not used.


    Link on Primary Key Indexes Whenever Possible

    To make queries run faster, you should have indexes on all fields in the query that join, restrict, or sort the data. Whenever possible, link on Primary Key fields instead of other fields. Indexes are most critical on tables with large numbers of records, so you may not see a difference on small tables. You also don’t need to add secondary indexes on fields that are part of referential integrity.


    Experiment With One-to-Many Restrictions

    If you have a one to many join in query with a restriction, try moving the restriction to the other side of the join. For example, if the restriction is on the many side, move it to the one side. Compare performance results for both versions, and choose the fastest one.


    De-Normalize If Necessary

    Although you should strive to normalize your data for best performance and future flexibility, you may want to consider denormalizing some of your data if you frequently run queries with joins would benefit from such data restructuring.


    Experiment With Sub Queries Instead Of Joins

    If you have a query with a join that is not performing adequately, consider replacing the join with a sub query. In some cases, the sub query may cause the overall query operation to run faster.


    Limit the Number of Fields Returned By Each Query

    Where possible, queries should use a Where clause to constrain, or limit, the number of records returned. This results in faster performance and reduced resource usage.

    Microsoft Access Form Optimization

    Save the SQL of the Form RecordSource as a Query

    We’ve seen situations where a saved query loads significantly faster than the same SQL string stored as the RecordSource of a form. Somehow, saved queries are optimized more than the SQL string behind the report.


    Close Unused Forms

    Close forms that aren’t being used. Every form that is open consumes memory that could be used by other parts of your applications.


    Open Forms Hidden

    Consider opening your application’s most commonly used forms when your application starts. Set their Visible properties to False, and then make the Visible as needed. This frontloads some performance hits to the application load event, making forms load faster when needed.


    Use the DataEntry Property of a Form

    If a form’s record source (the table or tables accessed by the form’s RecordSource property) contain a large number of records, and the form is primarily used to add new records, set the DataEntry property of the form to Yes. This precludes Access from having to retrieve existing records when the form loads.


    Don’t Sort A Form’s Recordset

    Avoid sorting records in a form’s underlying record source unless a particular presentation order is absolutely necessary for the form. This makes the form load faster.


    Base Forms on Queries-Minimize Fields Returned

    Base forms and subforms on queries rather than tables. By doing this, you can use the query to restrict the number of fields returned, making the form load faster.


    Use Lightweight Forms

    Consider replacing Visual Basic code in a form’s module with calls to standard modules, or with hyperlink objects. Then set the form’s HasModule property to False. This turns the form into a Lightweight form, making it load faster. Search Access online help for “Lightweight Forms” for more information. In Access 2007, you can use embedded macros for simple operations.


    Index Fields Used to Link SubForms to a Form

    Index all fields in the subform that are linked to the main form. Also index all fields in the subform that are used for criteria.


    Set Editing Properties on SubForms

    Set the subform’s AllowEdits, AllowAdditions, and AllowDeletions properties to No if the records in the subform aren’t going to be edited. Or set the RecordsetType property of the subform to Snapshot.


    Reduce the Number of Fields in ListBox and ComboBox Row Sources

    In the RowSource property of listbox and combobox controls, include only the fields that are necessary.


    Set AutoExpand on ComboBoxes to No

    Set the AutoExpand property of comboboxes to No if you don’t need the “fill in as you type” feature.


    First Field of an AutoExpand ComboBox Should Be Text

    In a combobox that has the AutoExpand property set to Yes, the first displayed field should be a Text data type instead of a Number data type. In order to find matches, Access needs to convert numeric values to text. If the data type is Text, this conversion can be skipped.


    Optimize Bound ComboBoxes

    If the bound field in a lookup combobox is not the displayed field, don’t use expressions for the bound field or the displayed field, don’t use restrictions (the WHERE clause) in the row source, and use single-table row sources wherever possible.


    Move Linked Data Local for ComboBox and ListBox Controls

    If the data that fills a list box or combo box does not change often, and that data comes from a linked table, consider moving that data’s table into the local database. This can be a huge performance boost, especially if the linked table is located on a network drive.


    Group Controls On Multiple Pages

    Consider grouping controls on multiple pages. When the form loads, prepare only the controls on the form’s first page. Defer operations on other page’s controls, such as setting the record source until the user moves to that page. This makes the form load faster.


    Only Load Subforms on Tab Pages when the Page is Selected

    If a tab control contains several pages (tabs) with subforms on them, the form will load quicker if the subforms on the tabs that aren’t visible aren’t loaded right away. Since those tab pages aren’t viewed yet, you can defer loading the data until the user clicks on the tab. You’ll take a performance hit the first time the user clicks on it, but if they never select the tab, you would never load the data. For more information, read Microsoft Access Performance Tip: Using Late Binding for Subforms on Tab Controls


    Close Forms That Contain Unbound OLE Objects

    Close forms that contain unbound OLE Objects when they are not in use. When you activate an unbound OLE objects, the memory used in that operation is not released until the form is closed.


    Convert Subforms to Listbox or Combobox Controls

    Where possible, convert subforms to listbox or combobox controls. It is far quicker to load a control than it is to load an additional form as a subform.


    Move Form Module Code to a Standard Module

    You can reduce a form’s load time by moving its code from the form module to a standard module. When the form loads, the form’s module doesn’t need to be loaded. Of course, the standard module needs to be loaded at some point, but once a standard module is loaded, it stays in memory until you close the database.


    Avoid Unnecessary Property Assignments

    Set only the properties that absolutely need to be set. Properties assignments can be relatively expensive in terms of performance. Review your form’s startup code to ensure that you are not setting any form or control properties that don’t need to be set.


    Use the Requery Method Instead of the Requery Action

    Use the Requery method instead of the Requery action. The method is significantly faster than the action.


    Give Visual Feedback

    Give the user some visual feedback during long operations. Consider using status meters to display a task’s progress. At a minimum, use the Hourglass cursor along with a status message.


    Keep Forms Lightweight With Hyperlinks

    Hyperlinks in label controls make it easy to open another Access object. So instead of placing command buttons on your forms to do common operations, investigate the possibility of using a label control with the Hyperlink properties. This approach eliminates the need for a command button, and its associated event code.


    Split Forms Into Multiple Pages

    Consider using multi-page forms, separated by the page-break character. This allows you to present only the controls needed, and can reduce form-load time. For example, if your form has 10 combobox controls that take a long time to fill, split the form into multiple pages using the PageBreak control. Then, pick the 5 combobox controls the user is most likely to use and place them on the first page. Place the remaining controls on the second page. Load time for the form should be substantially reduced, especially if the queries filling those combo box controls are complex.


    Minimize the Number of Controls

    Minimize the number of controls on your form. Loading controls is the biggest performance hit when loading a form.


    Avoid Overlapping Controls

    Avoid overlapping controls. It takes Access more time to render and draw controls that overlap each other than it does non-overlapping controls.


    Use Graphics Sparingly

    Use bitmap and other graphic objects sparingly as they can take more time to load and display than other controls.


    Use the Image Control

    Use the Image control instead of unbound object frames to display bitmaps. The Image control is a faster and more efficient control type for graphic images.

    Microsoft Access Report Optimizations

    Save the SQL of the Report RecordSource as a Query

    We’ve seen situations where a saved query loads significantly faster than the same SQL string stored as the RecordSource of a report. Somehow, saved queries are optimized more than the SQL string behind the report.


    Don’t Sort Report Queries

    Don’t base reports on queries that use an ORDER BY clause. Access reports use their Sorting and Grouping settings to sort and group records: the sort order of the underlying record set is ignored.


    Avoid Expressions and Functions In Sorting and Grouping

    Try to avoid reports that sort or group on expressions or functions.


    Index Fields Used In Sorting and Grouping

    Index any fields that are used for sorting or grouping.


    Base Reports on Queries-Minimize Fields Returned

    Base reports and subreports on queries instead of tables. By using a query, you can restrict the number of fields returned to the absolute minimum number, making data retrieval faster.


    Index Fields Used to Link Subreports

    Index all the fields used to link a subreport to a report.


    Index Subreport Fields Used for Criteria

    Index all subreport fields used for criteria. This will cause the subreport to link its records faster. Of course, remember that over-indexing can cause performance bottlenecks when editing, adding and deleting data.


    Avoid Domain Aggregate Functions in a Report’s RecordSource

    Do not use domain aggregate functions (such as DLookup) in a report’s recordsource property. This can have a significant performance impact on how long it takes the report to open and display pages.


    Use the NoData Event

    Use the report’s NoData event to identify empty reports. You can then display a message and close the report. This is easier than running a separate process to see if data exists for the report.


    Avoid Unnecessary Property Assignments

    Set only the properties that absolutely need to be set. Properties assignments can be relatively expensive in terms of performance. Review your form’s startup code to ensure that you are not setting any form or control properties that don’t need to be set.


    Eliminate Unnecessary Reports

    If a sub report is based on the same query as its parent report, or the query is similar, consider removing the sub report and placing its data in the main report. While this is not always feasible, such changes can speed up the overall report.


    Limit the Number of Controls on Reports

    Minimize the number of controls on your report. Loading controls is the biggest performance hit when loading a report.


    Avoid Overlapping Controls

    Avoid overlapping controls. It takes Access more time to render and draw controls that overlap each other than it does non-overlapping controls.


    Minimize Bitmap Usage

    Use bitmap and other graphic objects sparingly as they can take more time to load and display than other controls.


    Use the Image Control

    Use the Image control instead of unbound object frames to display bitmaps. The Image control is a faster and more efficient control type for graphic images.

    Macros Tips

    Convert Macros to Visual Basic Code

    Convert macros to Visual Basic code. In almost all cases, Visual Basic code runs faster than macros. This obviously doesn’t apply if you are using macros for a SharePoint deployment of your Access forms.

    Microsoft Access Module/VBA Performance Tips

    Make an MDE File

    If possible, make an MDE file out of your database. An MDE file cannot become decompiled, so your Visual Basic code always runs at top speed. Additionally, since no source code is stored in the MDE file, the database loads faster and uses less memory.


    Achieve the Compiled State

    Module code is saved in two states in your Access database: the source state, and the compiled state. The source state consists of the contents of your actual modules, with full text including white space, procedure and variable names, and comments. The compiled state is the executable version of your code. All comments and white space have been removed, and a stream of executable instructions has been produced-the code is ready to be run. The difference between these two states can cause your application to run slower than molasses in January if you don’t understand them.

    When you run a procedure, VBA checks to see if the module containing the procedure is compiled. If it is, VBA simply runs the code. If it is not compiled, VBA compiles it by loading the code into memory, performing a syntax check, and compiling it into an executable format. If all these operations succeed, it can then run the code. You probably surmised that this process of compiling is not a free lunch-it does take some time. And herein lies the crux of the matter: compiling code takes time, and compiling lots of code takes lots of time.

    So if you want your database to run as fast as possible, your task is obviously to reduce the amount of time Access spends compiling your code to a bare minimum. In fact, in an ideal application, all your code should be compiled and saved in the compiled state. So how do you go about this? Your Access database (or project in VBA parlance) is said to be in a compiled state when all modules, including form and report modules, are saved in both states in the database. This means that the original source code is stored, as is the compiled version. In such a state, Access runs much faster, because it can completely bypass the compilation process.

    Getting your database into the compiled state is actually rather easy:

    1. Open any module
    2. From the Debug Menu, select Compile and Save All Modules

    Your database is now in the compiled state. This includes form and report modules (called class modules using Access terminology) and standard modules. All VBA code that is called by your application is immediately ready for execution. There is no need for compilation. This is all fine and well, but is just as easy for your database to become decompiled. When you make certain changes to your database, it automatically becomes decompiled, which means that the compiled state that you created using the previous steps no longer exists.


    How to Avoid Decompilation

    So how do you avoid decompilation, or loss of the compiled state? Any of the following actions can decompile your database:

    • Modify any module code
    • Make changes to code-bearing objects, such as form, reports and controls, or create such code-bearing objects

    So the bottom line is: to avoid decompilation, don’t do the above. It’s not as bad as it seems. After all, your database does not need to be in a compiled state while you are doing development work on it-it only really requires the performance benefits of the compiled state when it is actually running on your user’s workstations. Therefore, if you follow these guidelines, you can enjoy peak performance from your module code:

    • During development, don’t use Compile All Modules. It is a waste of time, because the first time you make any changes to the module, it will decompile, or reverse the effect of Compile All Modules. Rather, use the Compile Loaded Modules option instead. This action only compiles the modules that are called by the modules you have open. This is a much quicker operation, and results in the same syntax checking that Compile All Modules does.
    • When you are ready to deliver your database for testing or live use, put it into the compiled state using the steps outlined above.

    Decompile to Eliminate Old VBA Compiled States

    The Compact feature only compacts the data, but not the code portion of an Access database. To flush the database of old code, shrink the size of your database, and avoid the infamous Bad DLL Calling Convention error, use the /decompile command occasionally. Read this reference for more information on the Microsoft Access decompile feature.


    ActiveX Controls Should Have References

    If you are using an ActiveX control, your database should have a Reference to the ActiveX (OCX) file. This allows you to use early binding to bind variables to the control’s objects, making it load and run faster. In most cases, this is handled for you: when you insert an ActiveX control into a form or report, Access automatically creates a Reference for that control.


    Use Option Explicit

    Always explicitly declare variables. Ensure this happens in every module in your application by using the Option Explicit phrase at the top of each module.


    Choose the Most Efficient Variable Type

    Use the most efficient variable type possible when declaring variables. For example, don’t use a Long Integer when an Integer will do. Avoid Variant types because the can be inefficient and slow.


    Use Early Binding

    Avoid using the Object data type. Instead use the specific data type for the object you are working with. This allows Visual Basic to employ “early binding” which can be substantially faster in many cases.


    Assign Things to Object Variables

    If you are going to refer to a property, control, object or data access object (DAO) more than once, assign it to an object variable.


    Use the Me Keyword

    Use the Me keyword instead of the Form!FormName form to refer to the form of a form’s module.


    Avoid the Immediate If Function if Parts Run Other Code

    Use the IIf (immediate if) statement sparingly. IIf() does not employ “short-circuit” evaluation. This means that both sides of the expression are always evaluated, which may not be what you want since intuitively it looks like only the criteria satisfying side would run.


    Use Dynamic Arrays

    Instead of fixed arrays, use dynamic arrays with the Erase and ReDim statements to make better use of memory.


    Take Advantage of Demand Loading

    Organize and structure you modules to take advantage of Visual Basic’s demand loading architecture. When a procedure is loaded from a module, the entire module is loaded into memory. By placing related procedures in the same module, you can reduce the number of loads the Visual Basic has to make.


    Eliminate Dead Code

    Eliminate unused procedures and unused variables. These elements use memory unnecessarily, and slow program load and execution. Our Total Access Analyzer program finds unused classes, procedures, variables, constants, enums, user defined types, and variables assigned but not used.


    Use Constants Instead of Variables

    If you are using data that is not going to change, put it in a constant instead of a variable. This allows Visual Basic to compile the value into the constant when the module is compiled, making the execution of that code faster.


    Avoid Infinite Recursion

    Avoid Infinite Recursion. Don’t have code that can call itself without having some type of short-circuit mechanism. This can lead to “Out of Stack Space” errors.


    Declare String Data Intelligently

    Visual Basic allocates stack and heap memory differently according to the type of strings you create. By understanding how this works, you can write more efficient string code. String variables in procedures that are non-static use space on the computer’s stack. Use the following information to write code that minimizes stack memory usage.

    • Local fixed-length strings less than or equal to 64 characters use 2 bytes for each character in the string. They don’t use heap memory.
    • Local fixed-length strings longer than 64 characters use 4 bytes of stack memory for a pointer to the variable in heap memory and 2 bytes of heap memory for each character in the string.Local variable-length strings use 4 bytes of stack memory for a pointer to the variable in heap memory, and a variable amount of heap memory according to the length of the string.
    • If your code used a large number of fixed-length strings of 64 characters or less, you can reduce stack usage by changing the strings to local variable-length strings or making them static fixed-length strings.

    Minimize OLE References

    Every time you reference a Visual Basic object, method or property, you are initiating one or more calls the OLE’s Idispatch interface. Each one of these calls takes time. Minimizing the number of such calls is one of the best ways to make you code run faster. You can minimize OLE references by doing the following:

    • Use object variables instead of directly referring to objects
    • Use the With statement and the For Each construct to minimize object references
    • Move references to properties and methods outside of loops
    • When you refer to a member of collection, do so with the object’s index number. Referring to a collections member with a name or expression introduces extra work, and therefore, more time.

    Turn Off Screen Painting

    Turn off screen painting during repetitive operations that update the screen. Consider using the Application.Echo property to turn off screen painting. Depending on the type of video card in your computer, this can have moderate to dramatic effects of performance.


    Don’t Write Code When a Query Would Be Better

    We’ve seen pages of VBA code written using various recordset operations when one or a few queries would do the trick. Queries are not only faster and optimizable, they’re easier to understand and maintain. If you’re not familiar with how to use Select, Update, Delete, and Append queries, learn them to best use Access or SQL Server.


    Close Your Database a Couple of Times a Day

    VBA dynamically loads code into memory as needed on a module level. If a function is called or a variable is used, the entire module containing that function or variable is loaded into memory. As you are developing your application, you keep loading code into memory. Visual Basic for Applications does not support dynamic unloading of these modules. Because of this, RAM will begin to fill up. To boost development performance (i.e. to decrease the amount of time you spend as a developer working on your application), you may want to close the database periodically to unload the modules. Note that you do not have to close Access itself, just the database itself. However, if you have library database code loaded, you should exit Access also.

    It is especially important to close your database after a Compile All Modules command. The Compile All Modules command pulls all of your code into memory. Closing and reopening the application will unload the code and enable you to develop faster because of the additional free memory.

    If you are developing your application in a single-user environment, you can improve your development performance by opening the application exclusively. This allows Visual Basic for Applications to save and compile faster by eliminating multiple-user save situations.


    ActiveX Controls Should Have References

    If you are using an ActiveX control, your database should have a Reference to the ActiveX (OCX) file. This allows you to use early binding to bind variables to the control’s objects, making it load and run faster. In most cases, this is handled for you: when you insert an ActiveX control into a form or report, Access automatically creates a Reference for that control.


    Don’t Use Expressions To Determine Loop Boundaries

    If use loop constructs in your VBA code, such as For…Next, Do…While, etc. don’t force VBA to evaluate the boundaries of the loop each time. For example, instead of saying:

    For intCounter = 0 To Forms.Count - 1
    ...
    Next intCounter			
    

    use this:

    intCount = Forms.Count - 1
    For intCounter = 0 To intCount
    ...
    Next intCounter
    

    In the second example, VBA only has to determine the value of Forms.Count once. In the first example, the value needs to be determined for each iteration of the loop.

    Data Access Objects (DAO) Programming Tips

    Use Seek Instead of Find

    Use Seek instead of Find… whenever possible. It uses indexes more efficiently and is the fastest data access method.

    Search Access Help for: Seek method


    Use Bookmarks For Record Navigation

    Whenever possible, use bookmarks to move among records instead of using the FindNext method. The Jet engine can navigate to bookmark values quicker than doing the sequential reads required by FindNext.


    Use Indexed Fields for FindRecord and FindNext

    If you can’t use the Seek method, and must use the FindRecord or FindNext methods, use them on indexed fields. These methods are much more efficient when used on a field that is indexed.

    Search Access Help for: Find methods


    Don’t Use Transactions Unless Necessary

    Microsoft Access lets you wrap table update code in transactions so you can rollback incomplete attempts to save data. Any code operation that adds, modifies, or deletes data can be enclosed in a transaction using the BeginTrans…CommitTrans pair.

    If you do not need to rollback your updates, you can avoid using transactions and the overhead of maintaining a rollback log.

    Jet Engine Tuning Tips

    Use SHOWPLAN To See Query Execution Plans

    Microsoft Jet implements a cost-based query optimizer in its query engine. During the compilation process of the query, Jet determines the most effective way to execute the query. You can view this plan using the ShowPlan registry setting.

    To use this setting, use the Registry Editor that comes with your operating system and add the following key to the registry:\\HKEY_LOCAL_MACHINE\SOFTWARE\MICROSOFT\Jet\3.0\Engines\Debug

    Under this key, add a string data type entry named JETSHOWPLAN in all capital letters. To turn ShowPlan on, set the value of this new entry to “ON”. To turn the feature off, set the value to “OFF”. When the feature is on, a text file called SHOWPLAN.OUT is created (or appended to if it already exists) in the current directory. This file contains the query plans.


    Tune Database Performance with Jet Registry Settings

    Microsoft Jet lets you tune many parameters to tweak the engine for the best possible performance. These registry keys are located in:

    HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Jet\3.5\Engines\Jet 3.5
    

    Also, you can use the DAO SetOption method to set these parameters at runtime. Using SetOption causes Jet to apply the changes immediately-the changes are not stored permanently in the registry, and you do not have to restart Access. If you want to get the maximum performance out of your data operations, experiment with these settings.

    For more information, search Access online help for “Customizing Windows Registry Settings for Data Access Objects.”


    Threads

    Experiment with the Threads setting in the registry. With this setting you can increase or decrease the number of operating system threads available to the Jet database engine.


    MaxBufferSize

    Use the MaxBufferSize registry setting to tune the data buffer used by the Jet database engine.


    UserCommitSync

    Use the UserCommitSync registry setting to tune performance of explicit transactions.


    ImplicitCommitSync

    Use the ImplicitCommitSync registry setting to tune the performance of implicit transactions.


    FlushTransactionTimeout

    Use the FlushTransactionTimeout registry setting to tune the performance of asynchronous write operations.


    ExclusiveAsyncDelay

    Use the ExclusiveAsyncDelay registry setting to tune the performance of asynchronous mode writes to exclusively opened databases.


    SharedAsyncDelay

    Use the SharedAsyncDelay registry setting to tune the performance of asynchronous mode writes to databases opened for shared access.


    PageTimeout

    Use the PageTimeout registry setting to change the delay for checking other user’s changes to the database.


    LockDelay

    Use the LockDelay registry setting to change how long Microsoft Jet waits between retries on locked pages in a shared database.


    MaxLocksPerFile

    Use the MaxLocksPerFile registry setting to tune performance by setting the maximum number of locks can be placed on a database (MDB) file. For more information, search Access online help for “Customizing Windows Registry Settings for Data Access Objects”, or visit this Microsoft Knowledgebase article 815281.


    RecycleLVs

    Use the RecycleLVs registry setting to determine how to memo, OLE and hyperlink data pages are recycled.


    Use ISAMStats To See Engine Detail

    Microsoft Jet contains an undocumented function called ISAMStats that shows various internal values. The syntax of the function is:

    ISAMStats ((StatNum As Long [, Reset As Boolean]) As Long
    

    where StatNum is one of the following values:

    0 – number of disk reads
    1 – number of disk writes
    2 – number of reads from cache
    3 – number of reads from read-ahead cache
    4 – number of locks placed
    5 – number of release lock calls

    For example, to see the number of disk reads, try the following code:

    Dim lngReads As Long
    lngReads = DBEngine.ISAMStats (0, False)
    Debug.Print lngReads
    

    To reset the number of disk reads back to 0, do this:

    lngReads = DDBEngine.ISAMStats (0, True)
    

    Microsoft Access Startup Tips

    Remove Code From Your Startup Form

    If you use a Startup form instead on an Autoexec macro, place the Visual Basic code needed for that form in the form’s module instead of a standard module. Since Access has to load your Startup form, it will automatically load that form’s module, which is generally faster than loading a standard module. This technique gives your application the appearance that it is loading faster.


    Minimize Startup Form Code

    Minimize the amount of code in your Startup form. You may want to defer certain operations, such as opening data access objects in code, or checking objects to a later time. This technique gives your application the appearance that it is loading faster.


    Don’t Use ActiveX Controls on Startup Form

    Don’t use ActiveX controls on your Startup Form. These controls can take long to load than other controls and will subsequently slow down the load time of your application.


    Automate the Deployment of the Latest Version of Your Database

    Centrally manage and simplify the automatic deployment of your Access application to each desktop. When you update your application, you want to distribute it quickly and easily. It’s also important to make sure each user is using the right version of Access. Our Total Access Startup program makes it easy to manage your database version and Access version by letting you distribute a simple shortcut to launch your program.

    Multiuser Performance Tips

    Split Your Database

    Split your database into an application and data database. Place only the data database on the server, keeping the application objects such as forms, reports and queries locally, where they can load and execute faster.


    Keep Static Data Local

    Keep static data, such as lookup tables, on the local machine. Update the local tables as necessary from the server. For example, a lookup table containing the two-letter abbreviations for American states is not likely to change anytime soon. When such a table is used in a data entry application, it is a performance bottleneck to retrieve that data from the server every time it is needed. Instead, copy that table to your application’s local database.


    Set Options to Avoid Lock Conflicts

    Avoid lock conflicts by setting the Refresh Interval, Update Retry Interval, Number of Update Retries, and ODBC Refresh Interval Settings in the Tools, Options menu.


    Tune Jet with the Registry

    Investigate the registry settings available for Access and Microsoft Jet for tuning data engine parameters. Search Access online help for “Performance” for more information on these settings

    Computer Performance Tips

    Increase RAM

    Increase the amount of RAM on your computer. Operating systems, Office, and other programs eat up a lot of memory. With the cost of memory so low, you should have at least 4 GB of RAM. With sufficient RAM, Access can perform its queries entirely in RAM without the need to write temporary data to disk.


    Avoid NTFS Compression

    If you are using NTFS compression under Windows NT, move your Access application to a non-compressed drive. NTFS adds a significant amount of overhead during disk operations, and Access applications are very disk-intensive.


    Make Sure Network Speed is Maximized

    Most networks and network cards support 10/100 Mbps standards. Make sure your machine is using the 100 and not the 10 if you’re accessing data across your network.


    Keep Enough Local Disk Space Free

    Access requires a fair amount of disk space to operate, especially with large databases. Operations such as running large action queries, adding lots of data, importing data, and compiling and saving module code can use a lot of additional space on a temporary basis. Additionally, transactions, and compacting the database can use a lot of disk space. A good rule of thumb is to have roughly 5-10 times the size of your largest database available in free local storage.

    Windows Performance Tips

    Keep Databases Local

    Whenever possible, keep your databases on a local drive instead of on a network drive. In almost all cases, local disk access is faster than network access.


    Install Access Locally

    Do not install Microsoft Access, or its related Microsoft Office components on a network. Most applications, including Access and Office perform better when run from local drives.


    Open Databases Exclusively

    If you are opening a database, and no other users need to use the database, open the database in exclusive mode. To do this, check the Exclusive checkbox in the Open Database dialog.


    Close Unneeded Applications

    Free up memory by closing applications that you aren’t using. If you are using the System Agent from the Microsoft Windows Plus Pack, consider turning it off to make more memory available for your access application.


    Optimize Your Disk

    Keep your disk in optimal shape by deleting unneeded files and emptying your Recycle Bin. Use a disk defragmenter such as the one found in the Windows 95 Accessories, System tools menu.


    Close Access Occasionally to Reclaim Leaked Memory

    Almost all Windows applications “leak” memory. This is due to data structures, variables, and other memory consumers that are not correctly released by the application. By closing Access, you allow Windows to reclaim this leaked memory.


    Install Windows Locally

    The Windows operating system should be installed on a local fixed drive on your computer. Because of the number of components involved in running Windows, a network installation of Windows causes poor performance, especially for database applications.

    Conclusion

    Microsoft Access has provided an incredibly powerful tool for the rapid development of database systems. With this power comes complexity—and complexity can lead to less than desirable performance. Use the tips in this paper to get your Access applications into top shape.

    https://www.fmsinc.com/microsoftaccess/Performance.html

  • How to shut down a custom Access application remotely

    Summary

    Sometimes, you may have to perform maintenance tasks on a Microsoft Access database, such as compacting or repairing, making backup copies, or making design modifications. Many of these operations require that all users exit the database. However, there is no built in way to force users to quit Microsoft Access. And it is not a good idea to just disconnect the user from Network solutions. That can cause the database to become corrupted.

    This article shows you one approach that you can use to gracefully shut down a front-end Access database application. You can also use many of these concepts to compact or repair the database, make backup copies, and so on.

    Microsoft provides programming examples for illustration only, without warranty either expressed or implied. This includes, but is not limited to, the implied warranties of merchantability or fitness for a particular purpose. This article assumes that you are familiar with the programming language that is being demonstrated and with the tools that are used to create and to debug procedures. Microsoft support engineers can help explain the functionality of a particular procedure, but they will not modify these examples to provide added functionality or construct procedures to meet your specific requirements.

    More information

    How the Solution Works

    The solution works as follow. On a server, there is a file in a folder. This file can be named anything. For this solution, a file named chkfile.ozx is used. When this file is renamed or deleted, it notifies the front-end Access application that it has to close.

    A form is built that opens when users start the front-end database application. This form checks for the existence of the file on the server at a set interval. It uses the TimerInterval property and the OnTimer event to do this.

    If the file is found, nothing happens. If the file is not found, the form will display another form to warn the user that the database will be automatically closed in a specified amount of time.

     Note

    This solution does not use the MsgBox function to warn the user. The MsgBox function will wait for user input before it runs any code. This would defeat the purpose of the solution.

    To gracefully close client sessions, this solution renames the file that is being checked. When everything that has to be accomplished is finished, this solution renames the file back to the original name. This lets the users know that they can start the front-end database again.

    This process can also be automated to provide unattended operation by using a scheduled service on the server that renames chkfile.ozx at specified times.

    Steps to Create a Sample Scenario

    To demonstrate how this solution works, you will have to have the following:

    • A folder on your hard disk with a path of C:\MyData.
    • An empty file. This solution will check for the presence of this file.
    • A split database design with the tables in a back-end database file and links to that table in the front-end database. The front-end database will contain the code that checks for the existence of the file at a set interval, and then warns the user.

    Creating the folder for the sample application

    Create a folder in the root directory of drive C, and name it MyData.

    Creating the empty text file

    1. Create a new text file in the MyData folder, and name it chkfile.txt.
    2. Rename the text file so that it has an extension of ozx (chkfile.ozx). When you are prompted, confirm that you want to change the file extension.

    Creating the back-end database

    1. Create a new database in the C:\MyData folder, and name it Northwind_Be.mdb.
    2. Import the Customers table from the Northwind sample database into the Northwind_Be.mdb database. (By default, Northwind is located in the C:\Program Files\Microsoft Office\Office10\Samples.)
    3. Close the database.

    Creating the front-end database

    1. Create another new database, and name it Northwind_Fe.mdb.
    2. Link the Customers table from the Northwind_Be.mdb database into the new Northwind_Fe.mdb
    3. Create an AutoForm based on the linked Customers table and save it as frmCustomers. Close this form.

    Creating the form with code that checks for the existence of the file

    1. Create an unbound form, and save it with the name frmAppShutDown. In a production database, this form would normally always be open but not visible. For this sample, it can be left open as usual.
    2. Set the TimerInterval property of the form to 60000 milliseconds. This is equal to one minute. (For your own solution, you can increase or decease this time interval.)
    3. In Microsoft Office Access 2003 or in earlier versions of Access, in Design view of the frmAppShutDown form, click Code on the View menu. In Microsoft Office Access 2007, in Design view of the frmAppShutDown form, click the Design tab, and then click View Code in the Tools group. Type or paste the following code:

    VBCopy

    Option Explicit
    Dim boolCountDown As Boolean
    Dim intCountDownMinutes As Integer
    
    Private Sub Form_Open(Cancel As Integer)
        ' Set Count Down variable to false
        ' on the initial opening of the form.
        boolCountDown = False
    End Sub
    
    Private Sub Form_Timer()
    On Error GoTo Err_Form_Timer
        Dim strFileName As String
        strFileName = Dir("c:\MyData\chkfile.ozx")
        If boolCountDown = False Then
            ' Do nothing unless the check file is missing.
            If strFileName <> "chkfile.ozx" Then
                ' The check file is not found so 
                ' set the count down variable to true and
                ' number of minutes until this session
                ' of Access will be shut down.
                boolCountDown = True
                intCountDownMinutes = 2
            End If
        Else
            ' Count down variable is true so warn
            ' the user that the application will be shut down
            ' in X number of minutes.  The number of minutes
            ' will be 1 less than the initial value of the
            ' intCountDownMinutes variable because the form timer
            ' event is set to fire every 60 seconds
            intCountDownMinutes = intCountDownMinutes - 1
            DoCmd.OpenForm "frmAppShutDownWarn"
            Forms!frmAppShutDownWarn!txtWarning = "This application will be shut down in approximately " & intCountDownMinutes & " minute(s).  Please save all work."
            If intCountDownMinutes < 1 Then
                ' Shut down Access if the countdown is zero,
                ' saving all work by default.
                Application.Quit acQuitSaveAll
            End If
        End If
    
    Exit_Form_Timer:
        Exit Sub
    
    Err_Form_Timer:
        Resume Next
    End Sub
    
    
    1. Save and then close the form.

    Creating the form that will serve to warn the user

     Note

    Do not use the MsgBox function to warn the user. The MsgBox function will wait for user input before it runs any code. This would defeat the purpose of the solution.

    1. Create an unbound form, and name it frmAppShutDownWarn. Add the following text box control:adocCopyName: txtWarning Type: Textbox
    2. Save and close the form.
    3. Create a macro that will open the frmCustomer form and the frmAppShutDown form at startup. Name the macro autoexec.
    4. Close and reopen the database.
    5. Rename chkfile.ozx to chkfile.old.

    Timing of Solution Events

     Note

    All of the following times are approximate, and they start after the renaming of chkfile.ozx.

    • One minute or less: Northwind_FE.mdb will notice that the file being checked is missing.
    • Two minutes: A form will be opened in Northwind_FE.mdb, notifying you that the database will close in one minute.
    • Three minutes: Northwind_FE.mdb will automatically close, and save all work.

    https://docs.microsoft.com/en-us/office/troubleshoot/access/shut-down-custom-application-remotely

    https://www.access-programmers.co.uk/forums/threads/remote-shutdown-of-a-shared-db.84557/

    https://www.access-programmers.co.uk/forums/threads/help-my-database-keeps-shutting-down.295846/#post-1546375

    http://www.peterssoftware.com/isd.htm

  • 7 Ways To Do Sequential Numbering in Microsoft Access

    Occasionally we come across project requirements that include the ability to do sequential numbering in Access on a set of data. We cannot just use the auto-number data type because there is no promise of sequentiality but only uniqueness. In other words, if someone creates a record then later deletes it, there will be a gap in the sequence. So, what are our options? There are actually several ways, each with its pros and cons.

    Sequential Numbering in Access Ben Clothier Microsoft Access Experts Chicago, IL (1)

    1. Let SQL Server Do It

    If you have a SQL Server backend, you could just use the windows function ROW_NUMBER() and define it as a part of a SQL Server view. This is probably the simplest, no-nonsense method that works well for any purposes. This was introduced in SQL Server 2005 so chances are high you can use it if you can create views.

    Swell, but I don’t have a SQL Server backend.

    In fact, we have had run into cases like this and we can still do it without the ROW_NUMBER() function. Let’s look at few sequential numbering methods that can be done entirely in Access…

    2. Use a Report with Running Sum Property

    If your objective is to generate a report, then you don’t want numbering to be a part of your query but rather a part of the report itself. You can use the “Running Sum” property of a textbox on a report to do all the work for you:

    RunningSumProperty

    3. …But I’m Exporting Raw Query Data

    The Running Sum method works great as long you are happy with creating and running an Access report, but not all requirements want the data in an Access report. Sometimes we do need to export a content of query and therefore we can’t use a report (or at least we don’t want extraneous formatting contents when we try to export a report).

    To do this entirely in a query, we need a bit of help from the VBA. We need to create a brand new VBA standard module and define a module-level variable, and two functions:
    Private lngRowNumber As Long

    Public Function ResetRowNumber() As Boolean
    lngRowNumber = 0
    ResetRowNumber = True
    End Function

    Public Function RowNumber(UniqueKeyVariant As Variant) As Long
    lngRowNumber = lngRowNumber + 1
    RowNumber = lngRowNumber
    End Function
    Then within your query’s SQL, you should call the functions as so:
    SELECT RowNumber(TablePrimaryKey) As RowNum
    FROM SomeTable
    WHERE ResetRowNumber()
    ORDER BY SomeColumn;

    Let’s look at why we have this setup. It seems strange to put a ResetRowNumber() call in a WHERE clause, doesn’t it? However, the WHERE clause is actually resolved prior to the SELECT clause. (For those who wants to geek out on SQL internals, Itzik Ben-Gan, a SQL Server MVP has a great post that outlines the logical query processing. Though this is specific to SQL Server, the Access database engine as well the majority of RBMS engines generally follow the same outline). This gives us a convenient point to ensure that the module level variable lngRowNumber is always correctly reset at the right time (e.g. before we start returning records from a query).

    Now, what’s the deal with the UniqueKeyVariant parameter for the RowNumber() function? We are not even using it all! True, but if we didn’t, Access will call RowNumber() only once for an entire query and thus we get “1” for the entire dataset. That is a good way to save on CPU processing – why call Now() for each row? In this case, we want to go against this CPU-saving measure and ensure that the RowNumber() is in fact called once per row to generate a new number for that row. Thus, we pass in a parameter — a primary key of a table is a good choice for this purpose. So we get a nice sequential number generated all in the query with VBA function.

    Finally, though it won’t be required, having a ORDER BY clause is probably going to be very essential. Without an explicit sort applied to query, you might find that sequential numbers are sequential… but to the wrong rows! It’s entirely up to you to define how it should be sorted so the sequential numbers make sense.

    4. Hey, I Opened This in a Datasheet and It’s All Messed Up!

    Ah, that’s the con – this works beautifully when you are in fact exporting data but when you are viewing the query’s output in a datasheet or a form, as you scroll around, Access will be re-evaluating the rows, including a call to the RowNumber(). But when it repeatedly calls RowNumber(), of course it keeps incrementing blindly, without any regards to whether a number was already generated for that row.

    So, if you wanted to use this idea in a form or datasheet, let me stop and first remind you – if this is going to be non-updatable, you can just embed a report as a subreport within a form and thus use Running Sum. But let’s discuss the case where you need to be able to edit data in the forms, even with sequential numbering generated for the data you are viewing. This means we need to be able to tie a sequential number to a specific row.

    The best way to do this is to use a primary key. In cases where a query is complex and joins several tables, we may need to use the primary key of the table that’s on the bottom of the relationship hierarchy or possible a concatenation of multiple tables’ unique key so that the expression will be unique and thus we can identify which sequential number we have assigned.

    We will then modify the mdlRowNumbers module accordingly:
    Private lngRowNumber As Long
    Private colPrimaryKeys As VBA.Collection

    Public Function ResetRowNumber() As Boolean
    Set colPrimaryKeys = New VBA.Collection
    lngRowNumber = 0
    ResetRowNumber = True
    End Function

    Public Function RowNumber(UniqueKeyVariant As Variant) As Long
    Dim lngTemp As Long

    On Error Resume Next
    lngTemp = colPrimaryKeys(CStr(UniqueKeyVariant))
    If Err.Number Then
    lngRowNumber = lngRowNumber + 1
    colPrimaryKeys.Add lngRowNumber, CStr(UniqueKeyVariant)
    lngTemp = lngRowNumber
    End If

    RowNumber = lngTemp
    End Function
    Basically, the only difference is that we now maintain a VBA.Collection, and this time we do make use of the UniqueKeyVariant parameter, using it as a key to locate the previously assigned number. If we don’t find it, we add to the VBA.Collection. The SQL to use those 2 functions remains identical. The downside is that if the query generates millions worth of rows, the memory may run out due to a large VBA.Collection. However, if a form is bound to that large dataset, it will likely have several other problems anyway so VBA.Collection should not be a problem for forms that loads a few hundred rows. The other downside is that VBA.Collection will persist indefinitely even long after the query has “finished.” Back to form example, you might need to remember to call the ResetRowNumber() function on a form’s close to ensure that you dispose of the VBA.Collection and free up the memory.

    5. But I Need it for Multiple Forms; I Can’t Have Several VBA.Collection Instances!

    The problem with the previous example is that it’s OK since you only need it in one place but if you will have several places where it is used, you now have to qualify the instance of VBA.Collection to each place – not insurmountable but too easy to get messy and hairy very fast.

    However, there is a property we can use to get ourselves out of this problem and that is the AbsolutePosition property of the recordset. We can create a textbox, bind it to expression “=RowNumber()” and then create a function on the module behind the form that references the recordset:
    Public Function RowNumber() As Variant
    On Error Resume Next
    If Me.NewRecord Then
    If Me.Dirty Then
    RowNumber = Null
    Exit Function
    End If
    End If

    With Me.RecordsetClone
    .Bookmark = Me.Bookmark
    RowNumber = .AbsolutePosition + 1
    End With

    If Err.Number Then
    RowNumber = Null
    End If
    End Function
    This requires no pre-knowledge of a primary key and only assumes that when you load the form initially, it is already sorted. Any records newly inserted will get the next bigger number. However, if you requery and it causes the newly inserted rows to be re-sorted, they will be now given a new number that relates to their actual position within the set, which can be either good or bad, depending on your specific requirements.

    You might find it useful to ensure that the RowNumber() is called after insert of records, perhaps with this code:
    Private Sub Form_AfterInsert()
    'Assumes there's a textbox named txtRowNumber
    'and it is bound to expression "=RowNumber()"
    Me.txtRowNumber.Requery
    End Sub

    You also will find the code much easier to re-use, perhaps by packaging into a class module and thus encapsulate the implementation while sharing among several forms without worrying about VBA.Collections bloating since you are just using an innate property of the Recordset object.

    6. Use the Auto-Number, Luke

    Yet another approach that does not require embedding VBA functions into your SQL is to just create a new table and insert data into it. That requires at least three SQL statements to do this:
    DROP TABLE tmpSomeTempTable;

    CREATE TABLE tmpSomeTempTable (
    RowID AUTOINCREMENT,
    SomeColumn TEXT(255)
    );

    INSERT INTO tmpSomeTempTable (
    SomeColumn
    )
    SELECT
    SomeColumn
    FROM tblSomeTable
    ORDER BY SomeColumn;
    Note this works only because we create a brand new table, add an autonumber column to it as well any other columns we need then insert records into it. It’ll be contiguous – as long we don’t delete any records from the table. Unfortunately, creating a brand new table every time we run this will result in bloat of the Access file – if you can do it in a separate Access file, all the better so that you can compact it when you need to.

    This also either requires 3 separate saved queries or 3 separate calls to DAO.Database.Execute methods, for each SQL statement, in addition to handling the error when the temporary table doesn’t exist. The simplest and clean method for this case could be this following VBA code:
    Dim db As DAO.Database
    Set db = CurrentDb()

    db.Execute strDropTableSQL ‘Do not use dbFailOnError parameter
    db.Execute strCreateTableSQL, dbFailOnError
    db.Execute strInsertDataSQL, dbFailOnError
    By omitting the dbFailOnError parameter only for the DROP TABLE statement, we won’t get a runtime error about a table not existing but get runtime errors if we can’t create the table or insert data for some reasons. I’m also not a big fan of creating multiple saved queries that are meant to be logically grouped together – it gets quite cluttered when there are several saved queries in a navigation pane like that. Keeping it all in VBA makes it clearer that the intention is to execute those statements together.

    7. Why Not Do it all in a Single SQL Statement? I Saw a Method that Does This…

    Yes, it is certainly possible to do it without VBA at all. One possible method is to write a subquery that counts all rows up to this row. This also assumes there is a primary key that you can use and you can define a sort order. Here’s a sample SQL:
    SELECT
    (
    SELECT COUNT(*)
    FROM SomeTable AS x
    WHERE x.PrimaryKey <= t.PrimaryKey
    ORDER BY x.SomeColumn
    ) AS RowNumber
    FROM SomeTable AS t
    ORDER BY t.SomeColumn;

    This will work OK for a small set of data, but the more data there is, the more intensive the subquery becomes, and it must be called repeatedly for each row. This is one of those rare cases where doing it in VBA is faster than doing it in SQL — after all, row numbering is fundamentally iterative, not set-based which just goes against the grain of what SQL was designed to do – solve a problem in a set-based fashion, non-iterative fashion.

    There are also other problems – for one thing, it’s not a very generic solution. You must customize several parts to work for different queries. If the set of tables used in the outermost FROM clause are different, to be the innermost FROM clause, you have to adjust the WHERE clause to reference the correct primary key (or some combination thereof). Then you have to also keep the ORDER BY in sync. That’s a lot of tweaks if you want to use it for different queries. That is why this is the last method listed here but it is one possible solution.

    So Many Ways to Number a Cat

    As you can see, an idea of sequential numbering can be solved many different ways using different domains. This is typical in this field of work and precisely why it pays to not just consider how you can do it in one domain but also whether the domain is the right domain. We saw how we can easily leverage built-in features such as Transact-SQL’s ROW_NUMBER() or Access report’s Running Sum property. We also not only saw how we can craft our custom solutions in either VBA or SQL but we also see how we can use both together to create a solution that is better.

    So in the end, it is good to look at the final goal first rather than the intermediate steps and think about the domains that might be better suited to get you there.

    From:

  • Is Not Null

    Put in where condition as below:

    Sub CreateRst_WithSQL()
       Dim conn As ADODB.Connection
       Dim myRecordset As ADODB.Recordset
       Dim strConn As String
    
       strConn = "Provider = Microsoft.Jet.OLEDB.4.0;" & _
          "Data Source=" & CurrentProject.Path & _
          "\mydb.mdb"
    
       Set conn = New ADODB.Connection
       conn.Open strConn
    
       Set myRecordset = conn.Execute("Select * from Employees WHERE ReportsTo IS NOT NULL ")
       Do Until myRecordset.EOF
           For Each fld In myRecordset.Fields
              Debug.Print fld.Name & "=" & fld.Value
           Next fld
           myRecordset.MoveNext
       Loop
    
       
       myRecordset.Close
       Set myRecordset = Nothing
       conn.Close
       Set conn = Nothing
    End Sub

    From:

    Is Not Null
  • Access combo box Set unique column but show other column and save other column

    1. Combo box : control souce -> unique column, but set first column to the column you want to show(Other Column)
    2. Other column: set hide control and set the value when combo box change
  • The Bang! (Exclamation Operator) in VBA

    One of the most peculiar aspects of Visual Basic for Applications in the ! operator, called the “bang”. There’s a lot of confusion about the bang operator, and nowhere have I seen the correct whole story about it. So, I present it here!

    The bang is particularly common in code for Microsoft Access, where you may see things like this:

    strCriteria = "EmployeeID = " & Me!txtEmplIDSet rst = CurrentDb.OpenRecordSet("SELECT * FROM Employees WHERE " & strCriteria)If Not rst.EOF Then    Me!txtName  = rst!Name    Me!txtTitle = rst!Title    Me!txtHireDate = rst!HireDateEnd Ifrst.CloseSet rst = Nothing

    Definition: The Bang (!) Operator

    What the bang operator does is simple and specific:

    The bang operator provides late-bound access to the default member of an object, by passing the literal name following the bang operator as a string argument to that default member.

    What it is NOT, in detail

    The bang operator is not a “late-bound dot”

    The most common explanation is that the “the difference between the dot operator and the bang operator is that the dot is early-bound and the bang is late-bound”. This is perpetuated in places such as the accepted answer to the StackOverflow question Bang Notation and Dot Notation in VBA and MS-Access or even a post on the Microsoft Office blog : Access Blog – Dot or Bang?.

    It is true that the dot is early-bound and the bang is late-bound, but they are not equivalent beyond that. The dot is the one and only way to access a named public member of an object. The bang causes the runtime to invoke the default member of an object and pass the name following the bang as a string argument. It looks like the bang is a “late-bound dot” when working with forms (as in the blog post linked above) because forms (and reports, and some other objects) have a default member called Controls which returns a reference to a control when given the name of a control in the form of a string. The result is that Me!someControl works just like Me.someControl … when working with a form or report.

    As a quick demonstration of this: Note that forms also have a number of built-in properties, such as WidthHeightFilter, etc. If the bang was a late-bound dot, then the following would work:

    123Sub Form_Load()    MsgBox “I am ” & Me!Width & ” twips wide”End Sub

    In fact, it does not work. It compiles fine because of course the bang is late-bound (there is no compile-time symbol validation). But at runtime the result is as if you had coded the following:

    1    MsgBox “I am ” & Me.Controls(“Width”) & ” twips wide”

    Of course there is no control that is called “Width”, so the code throws an error.

    The bang operator is not a collection accessor, or a call to “Item”

    You may also see the bang operator used against collection types, such as Recordset or plain Collection objects. In fact there is no requirement that the underlying object be a collection type, nor that the default method be called “Item”. It is merely convention that leads these things to normally be true.

    The one and only index accessor in VBA is the parenthesized expression, which confusingly is identical to the syntax for calling a function. In fact only arrays can be indexed in VBA. Object collections of all types accomplish the same-looking syntax by providing a default member that does in fact fetch a member of the collection. Put it this way: if VBA used square brackets for index access like all the C-style languages, the difference would become clear:

    123456789101112131415161718‘ **** Imaginary VBA with square bracket index access ****Dim arrLongs(5) As LongDim oRst As Recordset ‘ This would work because it is true index access:myLong = arrLongs[2] ‘ This would NOT work, because it is not actually index access:Set oNameField = Recordset[“Name”] ‘ This would still work:Set oNameField = Recordset(“Name”) ‘ … because it is really shorthand forSet oNameField = Recorset.Fields.Item(“Name”) ‘ … because Fields is the default member (a property) of’ Recordset, and Item is the default member (a function) of Fields

    Am I just being nit-picky? I don’t think so. The difference between true index access as with arrays and invoking an Item method is the same as the very real difference between directly accessing a public field of an object (a variable declared as Public) and invoking an Property Get method.

    What it is, in detail

    My definition above really says it all. To clarify I will present an example which demonstrates what it is by also showing what it is not, by constructing a simple class and then using the bang on it.

    Exported Module Code for BangDemo

    Note that flagging a default member in VBA is a little tricky because the IDE bundled with MS Office does not provide a way to set this flag. You can still do so by exporting the module, adding the required attribute manually, and re-importing the module. The following is the code for the BangDemo class as exported and modified to flag the default member. Save it in a file with the extension “cls” and import it to a VBA project in any Office application by choosing File > Import File from the menu in the Visual Basic editor.

    123456789101112131415161718192021VERSION 1.0 CLASSBEGIN  MultiUse = -1  ‘TrueENDAttribute VB_Name = “BangDemo”Option Explicit Public Function Item(ByVal Index) As Variant    Item = “I’m not a collection, but Index = ” & CStr(Index)End Function Public Function Foo(ByVal Arg) As Variant    Foo = “Foo! Arg = ” & CStr(Arg)End Function Public Function Frob(ByVal Index, _     Optional ByVal ExtraArg As Long = 42) As VariantAttribute Foo.VB_UserMemId = 0    Debug.Print “BangDemo.Frob : Invoked Frob”    Frob = “Frobbed ” & CStr(Index) & “, 0x” & Hex$(ExtraArg)End Function
    The test code

    Put the following code in a standard module, and run it (F5):

    12345Sub TestBang()    Dim oBangDemo As New BangDemo    Debug.Print “TestBang      : ” & oBangDemo!Foo    Debug.Print “TestBang      : ” & oBangDemo!BlahBlahBlahEnd Sub
    The result

    You will see the following printed to the Immediate window:

    1234BangDemo.Frob : Invoked FrobTestBang      : Frobbed Foo, 0x2ABangDemo.Frob : Invoked FrobTestBang      : Frobbed BlahBlahBlah, 0x2A
    What this shows
    • Not a member access operator: Note BangDemo has a method called Foo, but it is not invoked because the bang isn’t a member access operator. In addition, BangDemo doesn’t have any member called “BlahBlahBlah”, but oBangDemo!BlahBlahBlah works just fine, because again the bang isn’t a member access operator.
    • Not a collection accessor: BangDemo is not a collection of any kind, so clearly there are no “items” to access.
    • The name of the default member doesn’t matter: BangDemo does have a method called Item but that is not what is invoked by the bang because the bang doesn’t care what your methods are called. However the Frob method is invoked by the bang simply because it is marked as the default member.
    VBang-operator

  • MS Access Select top n query grouped by multiple fields

    SELECT StudentID, Year, Subject,  AVG(TestScore) AS AvgScore
      FROM
    (
      SELECT StudentID, Year, Subject, TestScore
       FROM MyTable t
       WHERE TestID IN
      (
       SELECT TOP 3 TestID 
         FROM MyTable
        WHERE StudentID = t.StudentID
          AND Year = t.Year
          AND Subject = t.Subject
        ORDER BY TestScore DESC, TestID
      )
    ) q
     GROUP BY StudentID, Year, Subject
     ORDER BY StudentID, Year, Subject;

    Sample output:

    | STUDENTID | YEAR | SUBJECT | AVGSCORE |
    |-----------|------|---------|----------|
    |         1 | 2012 |       1 |       91 |
    |         1 | 2012 |       2 |       84 |
    |         2 | 2012 |       1 |       94 |
    |         2 | 2012 |       3 |       95 |

    Data Source

    StudentID, Test ID, Score
    1,1, 95
    1, 2, 90
    1, 3, 90
    1, 4, 90
    2, 1, 99
    2, 2, 95
    2, 3, 90
    2, 4, 90

    https://stackoverflow.com/questions/21059077/ms-access-select-top-n-query-grouped-by-multiple-fields

  • 对数据进行分组的SQL

    Table:

    idscore
    156
    291
    367
    454
    556
    669
    761
    883
    999
    Subject_table

    MS Access

    SELECT Switch(score < 60,  "less 60",
            	score < 80,  "60 - 80",
            	score < 100 , "80-100"
            ) as score_type,count(id) as stu_cnt
            
            FROM subject_table 
    group by Switch(score < 60,  "less 60",
            	score < 80,  "60 - 80",
            	score < 100 , "80-100"
            )

    MySQL

    SELECT (case 
            	when score < 60 then "less 60"
            	when score < 80 then "60 - 80"
            	when score < 100 then "80-100"
            else "other"
            end) as score_type,count(id) as stu_cnt
            
            FROM `subject_table` 
            group by (case 
            	when score < 60 then "less 60"
            	when score < 80 then "60 - 80"
            	when score < 100 then "80-100"
            else "other"
            end)
            
  • transfering an export/import specification from one database to another (1 Viewer)

    In Access 2007, display the “External Data” tab. Click on the “Access” option.

    Use the Browse button to locate the file you want to import the saved import-export specification. Once a database has been specified, the “OK” button will become available. Clic the OK button to display the Import Objects dialog box. Click the “Options >>” button at the bottom of the list of buttons on the right side of the dialog box. One of the options in the “Import” group is “Import/Export Specs”. Check this option. When you click the “Ok” button on the Import Objects dialog box the specification will be imported.

    https://www.access-programmers.co.uk/forums/threads/transfering-an-export-import-specification-from-one-database-to-another.172896/