Sql选择查询与多列的位置

Question

我有一张简单的桌子

CREATE TABLE  a(
    id int IDENTITY(1,1) NOT NULL,
    x varchar(50) 
)

我发现以下查询有效

select cast (id as varchar(3))+cast (x as varchar(3)) c from a 
where cast (id as varchar(3))+cast (x as varchar(3))='1a'

但这不起作用

select cast (id as varchar(3))+cast (x as varchar(3)) c from a 
where c='1a'

有人知道那是为什么吗？请不要因为某些原因我不想使用

where id=1 and x ='a'

Answer 1

因为WHERE子句中的表达式被计算为在表达式之前限制行,所以在select-list中计算别名.

这是一个解决方法:

select aprime.*
from (select cast (id as varchar(3))+cast (x as varchar(3)) c from a) aprime
where aprime.c='1a';