本文介绍SQL中不同类型的联接操作，包括内联接、左联接、右联接及完全外部联接，并展示了如何使用 EXISTS 和 IN 进行子查询。这些技巧对于提高SQL查询效率至关重要。
--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')
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