select * from table where xm=
(select distinct xm from table)
----------------------------------------------
----------------------------------------------xm 唯一
select xm ,distinct dw from table where dw=
(select distinct dw from table)
----------------------------------------------
----------------------------------------------dw 唯一
(select distinct xm from table)
----------------------------------------------
----------------------------------------------xm 唯一
select xm ,distinct dw from table where dw=
(select distinct dw from table)
----------------------------------------------
----------------------------------------------dw 唯一
select * from table where exists (select distinct xm from table)是一种交集的概念