IDENTITY、SCOPE_IDENTITY、IDENT_CURRENT的分析

IDENT_CURRENT、IDENTITY、SCOPE_IDENTITY区别

概念解释

  • IDENT_CURRENT returns the last identity value generated for a specific table in any session and any scope.

  • IDENTITY returns the last identity value generated for any table in the current session, across all scopes.

  • SCOPE_IDENTITY returns the last identity value generated for any table in the current session and the current scope.

查找到有中文解释,但还是发现用英文解释,更容易理解。

IDENT_CURRENT,为指定表的所有会话和所有作用域生成的最后一个标识值;
IDENTITY,为当前会话的所有表和所有作用域的最后一个标识值;
SCOPE_IDENTITY,当前会话和当前作用域的所有表的最后一个标识值;

SQL说明

 

CREATE TABLE Test1(id int IDENTITY);
CREATE TABLE Test2(id int IDENTITY(100,1));
GO
CREATE TRIGGER Test1ins ON Test1 FOR INSERT 
AS
BEGIN
   INSERT Test2 DEFAULT VALUES
END;
GO
--End of trigger definition

SELECT id FROM Test1;
--IDs empty.
SELECT id FROM Test2;
--ID is empty.

--Do the following in Session 1
INSERT Test1 DEFAULT VALUES;
SELECT @@IDENTITY;
/*Returns the value 100. This was inserted by the trigger.*/

SELECT SCOPE_IDENTITY();
/* Returns the value 1. This was inserted by the 
INSERT statement two statements before this query.*/

SELECT IDENT_CURRENT('Test2');
/* Returns value inserted into Test2, that is in the trigger.*/

SELECT IDENT_CURRENT('Test1');
/* Returns value inserted into Test1. This was the INSERT statement four statements before this query.*/

 

所有在执行insert 表A、B、C的事务时候,取IDENT_CURRENT("A")的时候,又没有及时将此ident插入相应记录D,而这个记录表D又要求插入的ident有唯一索引。这时就可能出现唯一索引重复插入失败,因为,在执行事务的时候其它会话,可能已经将此ident插入记录表D。所以这时候用SCOPE_IDENTITY更稳健一点。

补充

 

sessionscope 在博客园找到的概念是这样的:

一个作用域 就是一个模块——存储过程、触发器、函数或批处理。因此,如果两个语句处于同一个存储过程、函数或批处理中,则它们位于相同的作用域中。

会话 一个用户连接产生的所有上下文信息。一个查询分析器窗口就是一个会话

但是一个用户connection代表一个session,这个好像不对。

A connection represents the external connection to the server (over a network or locally through shared memory for example).
A session represents a user process within SQL Server.
A connection may be associated with zero, one, or many sessions.

Take a look at the columns on sys.dm_exec_connections and sys.dm_exec_sessions. That should give you a feel for the differences.

 sys.dm_exec_connections http://msdn.microsoft.com/zh-cn/library/ms181509.aspx 

sys.dm_exec_sessions  http://msdn.microsoft.com/zh-cn/library/ms176013.aspx

 

 

posted @ 2013-03-18 23:31  VORO  阅读(1099)  评论(0编辑  收藏  举报