如何获得具有不同别名的两个表的并集

Question

我想获得联盟，但希望用不同的别名显示一列两次。 E.g

Select a as first from table1 
union 
select a as second from table1

首先显示为输出。

但我想展示第一和第二。如何在MSSQL中实现？

Answer 1

Union将两个查询的结果合并为一组列。没有办法让列名更改一半。我能想到的最接近的是

select a as first
, null as second 
from table1 

union all 

select null as first
, a as second 
from table1

Answer 2

您将拥有union all

的所有行

Select a as first from table1 
union all 
select a as second from table1

Answer 3

如果我做得对：

Select a as first,a as second from table1