sql连接查询问题

select t0*,t1.val2 from a t0 left jion b t1 on a.id=b.id
union
select t0*,t1.val2 from b t0 left jion a t1 on a.id=b.id

大概就是这样，你自己在看看左连接

select a.id,val1,val2
from
(select a.id,val1,val2 from a left join b on a.id=b.id
union
select a.id,val1,val2 from b left join a on a.id=b.id)
order by a.id

select id,val1,null from a
union
select id,null,val2 from b

--> -->

if not object_id('Tempdb..#a') is null
drop table #a
Go
Create table #a([id] int,[val1] nvarchar(1))
Insert #a
select 1,N'a' union all
select 3,N'b'
Go
--> -->

if not object_id('Tempdb..#b') is null
drop table #b
Go
Create table #b([id] int,[val2] nvarchar(1))
Insert #b
select 1,N'a' union all
select 2,N'b' union all
select 4,N'd'
Go
select ID,max(val1)as val1,max(val2)