RowVersion数据类型

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RowVersion数据类型是系统自动生成的，唯一的，二进制数字，数值和binary(8)相同，RowVersion通常用作给Table的数据行加版本戳，存储大小为 8 个字节。RowVersion数据类型只是永恒递增的数字，不保留日期或时间，但是可以使用RowVersion来比较数据行更新时间的先后，如果@rv1<@rv2，那么表明@rv2的更新发生在@rv1之后。

每个数据库都只有一个自增的计数器（Counter），每次对拥有RowVersion 字段的Table执行Insert或Update命令，该计数器都会增加。一个Table最多有一个RowVersion 字段，只要对Table执行Insert或Update命令，该字段就会被更新为计数器（Counter）的最新值。

Each database has a counter that is incremented for each insert or update operation that is performed on a table that contains a rowversion column within the database. This counter is the database rowversion. This tracks a relative time within a database, not an actual time that can be associated with a clock. A table can have only one rowversion column. Every time that a row with a rowversion column is modified or inserted, the incremented database rowversion value is inserted in the rowversion column. 

RowVersion字段的特性：

1. 由于每个数据库只有一个Counter，因此，RowVersion的值在当前数据库中是唯一的，所有拥有RowVersion字段的Table，该字段的值都是不同的；
2. RowVersion 只会递增，不会回滚；如果数据的更新（Insert或Update）的事务回滚，RowVersion字段不会回滚；
3. TimeStamp 已过时，避免用于产品设计中，使用RowVersion代替；

在 Table 中增加RowVersion字段，能够检查该行是否被更新（insert或update），如果当前值跟最近一次记录的RowVersion值不同，说明该数据行被更新过。

You can use the rowversion column of a row to easily determine whether any value in the row has changed since the last time it was read. If any change is made to the row, the rowversion value is updated. If no change is made to the row, the rowversion value is the same as when it was previously read. 

一，查看当前数据库的RowVersion

1，全局变量@@DBTS用于返回当前数据库的RowVersion，@@DBTS 返回值的数据类型是varbinary(8)。

@@DBTS value is not rolled back when a transaction rolls back or when an INSERT or UPDATE query causes an error.

@@DBTS returns the value of the current rowversion data type for the database. The rowversion is guaranteed to be unique in the database. @@DBTS returns a varbinary which is the last-used rowversion value of the current database. A new rowversion value is generated when a row with a rowversion column is inserted or updated. Any INSERT, UPDATE and CREATE queries will internally increment the rowversion values.

2，非确定性函数 MIN_ACTIVE_ROWVERSION() 用于返回当前数据库的下一个RowVersion值，其值是@@DBTS+1。

MIN_ACTIVE_ROWVERSION is a non-deterministic function that returns the lowest active rowversion value in the current database. A new rowversion value is typically generated when an insert or update is performed on a table that contains a column of typerowversion. If there are no active values in the database, MIN_ACTIVE_ROWVERSION returns the same value as @@DBTS + 1.

select @@DBTS as dbts,MIN_ACTIVE_ROWVERSION() as min_active

二，示例

1，查看数据库当前的RowVersion

declare @rv rowversion
set @rv=@@DBTS
select @rv as rv

2，创建含有RowVersion列的表，并插入数据

create table dbo.dt_rv
(
id int not null,
rv rowversion not null
)

insert into dbo.dt_rv(id)
values(1)

3，查看当前的RowVersion和表中的数据

select id,rv,@@dbts as dbts 
from dbo.dt_rv

4，更新表，查看RowVersion值的变化

update dbo.dt_rv 
set id=2 
where id=1

select id,rv,@@dbts as dbts 
from dbo.dt_rv

 5，测试事务rollback时，RowVersion值的变化

begin tran 

insert into dbo.dt_rv(id)
values(1)

rollback tran 

select id,rv,@@dbts as dbts 
from dbo.dt_rv

当事务回滚时，RowVersion字段的值不会回滚，RowVersion字段的值只会递增。

参考文档：

rowversion (Transact-SQL)

RowVersion数据类型

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原文地址：http://www.cnblogs.com/ljhdo/p/4791838.html