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: 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 …
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.…
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…
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 …
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…
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 …