@@identity、scope_identity()、IDENT_CURRENT('tablename')函数的区别

@@IDENTITY 和SCOPE_IDENTITY 返回在当前会话中的任何表内所生成的最后一个标识值。但是，SCOPE_IDENTITY 只返回插入到当前作用域中的值；@@IDENTITY 不受限于特定的作用域。

例如，有两个表 T1 和 T2，并且在 T1 上定义了 INSERT 触发器。当将某行插入 T1 时，触发器被激发，并在 T2 中插入一行。 该方案演示了两个作用域：在 T1 上的插入，以及在 T2 通过触发器的插入。

假设 T1 和 T2 都有标识列，@@IDENTITY 和 SCOPE_IDENTITY 将在 T1 上的 INSERT 语句的最后返回不同的值。@@IDENTITY 将返回在当前会话中的任何作用域内插入的最后一个标识列的值。这是在 T2 中插入的值。SCOPE_IDENTITY() 将返回在 T1 中插入的 IDENTITY 值。这是在同一个作用域内发生的最后的插入。如果在任何 INSERT 语句作用于作用域中的标识列之前调用 SCOPE_IDENTITY() 函数，则该函数将返回空值。

如果语句和事务失败，它们会更改表的当前标识，从而使标识列中的值出现不连贯现象。即使未提交试图向表中插入值的事务，也永远无法回滚标识值。例如，如果因 IGNORE_DUP_KEY 冲突而导致 INSERT 语句失败，表的当前标识值仍然会增加。

以下示列创建两个表，TZ 和 TY，并在 TZ 中创建一个 INSERT 触发器。当将某行插入表 TZ 中时，触发器 (Ztrig) 将激发并在 TY 中插入一行。

USE tempdb
GO
CREATE TABLE TZ (
   Z_id  int IDENTITY(1,1)PRIMARY KEY,
   Z_name varchar(20) NOT NULL)

INSERT TZ
   VALUES ('Lisa')
INSERT TZ
   VALUES ('Mike')
INSERT TZ
   VALUES ('Carla')

SELECT * FROM TZ

--Result set: This is how table TZ looks.
Z_id   Z_name
-------------
1      Lisa
2      Mike
3      Carla

CREATE TABLE TY (
   Y_id  int IDENTITY(100,5)PRIMARY KEY,
   Y_name varchar(20) NULL)

INSERT TY (Y_name)
   VALUES ('boathouse')
INSERT TY (Y_name)
   VALUES ('rocks')
INSERT TY (Y_name)
   VALUES ('elevator')

SELECT * FROM TY
--Result set: This is how TY looks:
Y_id  Y_name
---------------
100   boathouse
105   rocks
110   elevator

/*Create the trigger that inserts a row in table TY
when a row is inserted in table TZ*/
CREATE TRIGGER Ztrig
ON TZ
FOR INSERT AS
   BEGIN
   INSERT TY VALUES ('')
   END

/*FIRE the trigger and determine what identity values you obtain
with the @@IDENTITY and SCOPE_IDENTITY functions.*/
INSERT TZ VALUES ('Rosalie')

SELECT SCOPE_IDENTITY() AS [SCOPE_IDENTITY]
GO
SELECT   @@IDENTITY AS [@@IDENTITY]
GO

下面是结果集：

SCOPE_IDENTITY
4
/*SCOPE_IDENTITY returned the last identity value in the same scope. This was the insert on table TZ.*/

@@IDENTITY
115
/*@@IDENTITY returned the last identity value inserted to TY by the trigger. This fired because of an earlier insert on TZ.*/

http://www.cnblogs.com/MingDe/archive/2011/10/12/2208749.html

USE [tempdb]
GO

CREATE TABLE [dbo].[MainTB](
    [Id] [int] IDENTITY(1,1) NOT NULL,
    [Name] [nvarchar](50) NULL,
 CONSTRAINT [PK_MainTB] PRIMARY KEY CLUSTERED
(
    [Id] ASC
)WITH (PAD_INDEX  = OFF, STATISTICS_NORECOMPUTE  = OFF, IGNORE_DUP_KEY = OFF, ALLOW_ROW_LOCKS  = ON, ALLOW_PAGE_LOCKS  = ON) ON [PRIMARY]
) ON [PRIMARY]

GO

CREATE TABLE [dbo].[SubTB](
    [Id] [int] IDENTITY(100,5) NOT NULL,
    [Memo] [nvarchar](50) NULL,
    [MainKeyId] [int] NOT NULL,
 CONSTRAINT [PK_SubTB] PRIMARY KEY CLUSTERED
(
    [Id] ASC
)WITH (PAD_INDEX  = OFF, STATISTICS_NORECOMPUTE  = OFF, IGNORE_DUP_KEY = OFF, ALLOW_ROW_LOCKS  = ON, ALLOW_PAGE_LOCKS  = ON) ON [PRIMARY]
) ON [PRIMARY]

GO

ALTER TABLE [dbo].[SubTB]  WITH CHECK ADD  CONSTRAINT [FK_SubTB_MainTB] FOREIGN KEY([MainKeyId])
REFERENCES [dbo].[MainTB] ([Id])
GO

ALTER TABLE [dbo].[SubTB] CHECK CONSTRAINT [FK_SubTB_MainTB]
GO

----------------------------------------------------

INSERT INTO dbo.MainTB(Name)
SELECT '張三'

SELECT SCOPE_IDENTITY() AS [SCOPE_IDENTITY]
SELECT @@IDENTITY AS [@@IDENTITY]
SELECT IDENT_CURRENT('MainTB')
SELECT IDENT_CURRENT('SubTB')

----------------------------------------------------

INSERT INTO dbo.MainTB(Name)
SELECT '李四'
UNION ALL
SELECT '王五'

SELECT SCOPE_IDENTITY() AS [SCOPE_IDENTITY]
SELECT @@IDENTITY AS [@@IDENTITY]
SELECT IDENT_CURRENT('MainTB')
SELECT IDENT_CURRENT('SubTB')

----------------------------------------------------

INSERT INTO dbo.MainTB(Name)
SELECT '李四'
UNION ALL
SELECT '王五'

INSERT INTO dbo.SubTB(Memo)
SELECT '備註一'
UNION ALL
SELECT '備註二'

SELECT SCOPE_IDENTITY() AS [SCOPE_IDENTITY]
SELECT @@IDENTITY AS [@@IDENTITY]
SELECT IDENT_CURRENT('MainTB')
SELECT IDENT_CURRENT('SubTB')

-----------------------------------------------------------
INSERT INTO dbo.MainTB(Name)
SELECT '李四'
UNION ALL
SELECT '王五'

INSERT INTO dbo.SubTB(Memo,MainKeyId)
SELECT '備註一',1
UNION ALL
SELECT '備註二',1

SELECT SCOPE_IDENTITY() AS [SCOPE_IDENTITY]
SELECT @@IDENTITY AS [@@IDENTITY]
SELECT IDENT_CURRENT('MainTB')
SELECT IDENT_CURRENT('SubTB')
-----------------------------------------------------------
CREATE TRIGGER trgTEST
ON MainTB
FOR INSERT AS
   BEGIN
   INSERT INTO dbo.SubTB(Memo,MainKeyId) SELECT '備註一',SCOPE_IDENTITY()
   END

INSERT INTO dbo.MainTB(Name)
SELECT '李四'

SELECT SCOPE_IDENTITY() AS [SCOPE_IDENTITY]
SELECT @@IDENTITY AS [@@IDENTITY]
SELECT IDENT_CURRENT('MainTB')
SELECT IDENT_CURRENT('SubTB')
------------------------------------------------------

INSERT INTO dbo.MainTB(Name)
SELECT '李四'
WAITFOR DELAY '00:00:20'
SELECT SCOPE_IDENTITY() AS [SCOPE_IDENTITY]
SELECT @@IDENTITY AS [@@IDENTITY]
SELECT IDENT_CURRENT('MainTB')
SELECT IDENT_CURRENT('SubTB')
SELECT * FROM dbo.MainTB
SELECT * FROM dbo.SubTB

--新开查询窗同时执行以下Script
INSERT INTO dbo.MainTB(Name)
SELECT '李四'
SELECT SCOPE_IDENTITY() AS [SCOPE_IDENTITY]
SELECT @@IDENTITY AS [@@IDENTITY]
SELECT IDENT_CURRENT('MainTB')
SELECT IDENT_CURRENT('SubTB')
SELECT * FROM dbo.MainTB
SELECT * FROM dbo.SubTB

IDENT_CURRENT('MainTB'):取表的最后编号，注意如果在高并发下，可能会取的不是预期的结果。