表表达式和CTE

 

-- =============================================            
-- Author:      余波(杭州)            
-- Create date: 2011/09/27            
-- Description: 表表达式和CTE           
-- =============================================

--表a1结构及数据如下
/*
id	name
1	2
2	3
3	4
*/
---表表达式，外部指定列别名
select idname from (select ID from a1)as d(idname)

---表表达式，内部指定列别名
select idname from (select id as idname from a1)d

---表表达式不支持多引用
select * from (select id from a1)t join (select id from a1)t on t.id=t.id  ---错误
select * from (select id from a1)t join (select id from a1)u on u.id=t.id  ---正确

----cte中with前面最好用分号的原因是：with在T-SQL中还有其它用途，为避免歧义CTE with
----之前的语句必须用分号结束(但T-SQL并未强制要求)

----CTE和表表达式相同，也可以再内部和外部指定列别名
----内部
with cte1 as
(
	select id as idname from a1
)select idname from cte1;

----外部
with cte2(idname) as
(
	select id from a1
)select * from cte2

---多cte查询，最后可以引用上面任意一个cte
;with cte3 as
(select id from a1)
,
cte4 as
(
	select id from cte3 where id>=2
)
select * from cte3 a left join cte4 b on a.id=b.id