sqlserver存储过程中SELECT 与 SET 对变量赋值的区别

<P style="LINE-HEIGHT: normal">sql Server推荐使用 SET 而不是 SELECT 对变量进行赋值。<BR style="LINE-HEIGHT: normal">当表达式返回一个值并对一个变量进行赋值时，推荐使用 SET 方法。<BR style="LINE-HEIGHT: normal">下表列出 SET 与 SELECT 的区别。请特别注意红色部分。

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normal; FONT-FAMILY: Arial; WORD-WRAP: break-word; VISIBILITY: visible !important; FONT-SIZE: 12px">	normal; FONT-FAMILY: Arial; WORD-WRAP: break-word; VISIBILITY: visible !important; FONT-SIZE: 12px"> normal">set	normal; FONT-FAMILY: Arial; WORD-WRAP: break-word; VISIBILITY: visible !important; FONT-SIZE: 12px"> normal">select
normal; FONT-FAMILY: Arial; WORD-WRAP: break-word; VISIBILITY: visible !important; FONT-SIZE: 12px">同时对多个变量同时赋值	normal; FONT-FAMILY: Arial; WORD-WRAP: break-word; VISIBILITY: visible !important; FONT-SIZE: 12px">不支持	normal; FONT-FAMILY: Arial; WORD-WRAP: break-word; VISIBILITY: visible !important; FONT-SIZE: 12px">支持
normal; FONT-FAMILY: Arial; WORD-WRAP: break-word; VISIBILITY: visible !important; FONT-SIZE: 12px">表达式返回多个值时	normal; FONT-FAMILY: Arial; WORD-WRAP: break-word; VISIBILITY: visible !important; FONT-SIZE: 12px">出错	normal; FONT-FAMILY: Arial; WORD-WRAP: break-word; VISIBILITY: visible !important; FONT-SIZE: 12px">normal; COLOR: rgb(255,0)">将返回的最后一个值赋给变量
normal; FONT-FAMILY: Arial; WORD-WRAP: break-word; VISIBILITY: visible !important; FONT-SIZE: 12px">表达式未返回值	normal; FONT-FAMILY: Arial; WORD-WRAP: break-word; VISIBILITY: visible !important; FONT-SIZE: 12px">变量被赋null值	normal; FONT-FAMILY: Arial; WORD-WRAP: break-word; VISIBILITY: visible !important; FONT-SIZE: 12px">normal; COLOR: rgb(255,0)">变量保持原值

normal">下面以具体示例来说明问题：
normal">create table chinadba1(
normal">userid int,
normal">addr varchar(128)
normal">)
normal">go
normal">insert into chinadba1(userid,addr) values(1,'addr1')
normal">insert into chinadba1(userid,addr) values(2,'addr2')
normal">insert into chinadba1(userid,addr) values(3,'addr3')
normal">go
normal">
normal">表达式返回多个值时，使用 SET 赋值
normal">declare @addr varchar(128)
normal">set @addr = (select addr from chinadba1)
normal">/*
normal">--出错信息为
normal">服务器: 消息 512，级别 16，状态 1，行 2
normal">子查询返回的值多于一个。当子查询跟随在 =、!=、<、<=、>、>= 之后，或子查询用作表达式时，这种情况是不允许的。
normal">*/
normal">go
normal">表达式返回多个值时，使用 SELECT 赋值 declare @addr varchar(128)
normal">select @addr = addr from chinadba1
normal">print @addr --结果集中最后一个 addr 列的值
normal">--结果: addr3
normal">go
normal">
normal">表达式未返回值时，使用 SET 赋值 declare @addr varchar(128)
normal">set @addr = '初始值'
normal">set @addr = (select addr from chinadba1 where userid = 4 )
normal">print @addr --null值
normal">go
normal">
normal">表达式未返回值时，使用 SELECT 赋值 declare @addr varchar(128)
normal">set @addr = '初始值'
normal">select @addr = addr from chinadba1 where userid = 4
normal">print @addr --保持原值
normal">go
normal">
normal">需要注意的是，SELECT 也可以将标量子查询的值赋给变量，如果标量子查询不返回值，则变量被置为 null 值。
normal">此时与 使用 SET 赋值是完全相同的
normal">对标量子查询的概念大家应该都觉得陌生，举个例子就能说明
normal">declare @addr varchar(128)
normal">set @addr = '初始值'
normal">--select addr from chinadba1 where userid = 4 为标量子查询语句
normal">select @addr = (select addr from chinadba1 where userid = 4)
normal">print @addr --null值
normal">go

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