级联查询 EXISTS与 IN互换

--联接
SELECT
FROM dbo.k_baoming INNER JOIN
      dbo.k_kebiao ON dbo.k_baoming.o_flowid = dbo.k_kebiao.f_apply_id

--左向外联接
SELECT dbo.k_baoming.o_flowid
FROM dbo.k_baoming LEFT OUTER JOIN
      dbo.k_kebiao ON dbo.k_baoming.o_flowid = dbo.k_kebiao.f_kebiaoID

--右向外联接
SELECT dbo.k_baoming.o_flowid
FROM dbo.k_baoming RIGHT OUTER JOIN
      dbo.k_kebiao ON dbo.k_baoming.o_flowid = dbo.k_kebiao.f_kebiaoID

--完整外部联接
SELECT dbo.k_baoming.o_flowid
FROM dbo.k_baoming FULL OUTER JOIN
      dbo.k_kebiao ON dbo.k_baoming.o_flowid = dbo.k_kebiao.f_kebiaoID

--可以不要OUTER

 

--EXISTS 与 IN 互换
SELECT * FROM Article WHERE classid in(SELECT ID FROM class WHERE parentID='219')
go
SELECT * FROM Article WHERE exists(SELECT ID FROM class WHERE class.id=Article.classid AND class.parentID='219')

 

IN适合于外表大而内表小的情况；EXISTS适合于外表小而内表大的情况。