declare @t1 table ( id int , val varchar(20))
insert into @t1
select 1 , 'a'
union select 2 , 'b'
union select 3 , 'd' ;
declare @t2 table ( id int , val varchar(20))
insert into @t2
select 2 , 'b'
union select 3 , 'b'
union select 4 , 'c' ;
select * from @t1 ;
select * from @t2 ;
select *
from @t1 t1
left join @t2 t2 on ( t1.val = t2.val)
LeftJoin 扫描两个结果集，对外表来说是 左行数 个表扫描。