-- =============================================-- Author:            <Author,,CC>-- Create date:        <Create Date,, 2014-06-29 12:25:20.010>-- Description:        <Description,,加别名的重要性>-- Environment:        <Version,, Microsoft SQL Server 2008 R2 (RTM) - 10.50.1600.1 (X64))>-- =============================================use testgocreate table t1(id int ,id1 int)create table t2(id11 int,id2 int)insert into t1(id ,id1)select 1,2 union allselect 2,3 union allselect 3,4 union allselect 4,5 union allselect 5,5insert into t2(id11,id2) select 1,2 union allselect 2,3create table t3(id int,id2 int) insert into t3(id,id2) select 1,2 union allselect 2,3-- 1.查询t1表中的id在t3中的记录,没有别名，记录正确select * from t1 a where id in (select id from t3 b)  --2. 假如in的内查询中不存在外查询的列select * from t1 awhere id in (select id from t2 b)/*此时我们可以看到语句并没有错，而是会去寻找t1中的数据,语句结果同下面的查询效果：select * from t1 a where id in (select a.id from t2)其实是内查询不存在，所以结果就自己去寻找外面的查询*/--3. 我们把内表加上别名,此时才是我们要的结果,并且提示报错select * from t1 awhere id in (select b.id from t2 b)--删除t2中的数据，再测试一下truncate table t2--3. t2中没有数据，此时则不会往外层寻找select * from t1 a where id in (select a.id from t2)drop table t1,t2,t3