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Monthly Archives: November 2011

How to clear dns cache?

30-Nov-11

In Microsoft Windows: In Microsoft Windows, you can use the command ipconfig /flushdns to flush the DNS resolver cache.
– Start -> Run -> type cmd – In command prompt, type C:>ipconfig /flushdns

In Mac OSX: In Mac OSX, you …

Recursive Relationships

15-Nov-11

Recursive relationships: relationships exist between entity instances of the same type.

One-to-one: If we were to track which employees were married to other employees, we would expect each to be married to either zero or one other employee at any …

多对多必须通过单独的一张表来表示

15-Nov-11

如班级和教师的关系由三张表来表示:班级表,教师 及 班级教师表

In particular, the logical model does not contain any many-to-many relationships. There is a simple rationale for this difference – relational databases do not directly support many-to-many relationships, so they must be transformed using an intersection entity.…

About Count

11-Nov-11

select count(*) FROM [AdventureWorks].[Person].[Contact]
返回表中所有的记录的个数

select COUNT(MiddleName) from [AdventureWorks].[Person].[Contact]
返回字段中,值非空的记录的个数(重复的也算进去的) 

select COUNT(distinct MiddleName) from [AdventureWorks].[Person].[Contact]
返回字段中不重复且非空的记录的个数

Result:
19972
11473
70…

Cursors in SQL

06-Nov-11

SQL was designed as a set-oriented processing language. Some business rules( or poor physical design) require performing actions on row-by-row basis. Consider the following example:
. Increase the price of books <=$15 by 15%
. Decrease the price of books …

View – With Check Option

02-Nov-11

Creating a view using With Check Option will restrict the queries to only those rows directly visible by the view.

http://www.sqlteam.com/FORUMS/topic.asp?TOPIC_ID=66019
It prevents row from dissappearing from the view implementing this option.
drop table  T100
go
drop view  VT100
go…

Basic Rules of Normalization

01-Nov-11

Without going into a dissertation on data modeling(itself a subject of many full-length treatises), tables should follow basic rules of normalization:

Avoid duplicate data, avoid repeating groups in tables, and only put data in tables where the information is directly