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  • mysql查询(四)

    查询存在" A001 "课程但可能不存在" A002 "课程的情况(不存在时显示为 null

    成绩表为:ex_chengjibiao_95

    (1)查询A001

    命令:SELECT * from ex_chengjibiao_95 WHERE CId='A001'

    (2)查询A002

    命令:SELECT * from ex_chengjibiao_95 WHERE CId='A002'

    (3)满足存在A001,但是未必存在

             满足A001方向结果,未必满足A002方向结果

    命令:SELECT a.Sid,a.Cid,a.score,b.Cid,b.score from

    (SELECT * from ex_chengjibiao_95 WHERE Cid='A002')as a

    left join

    (SELECT * from ex_chengjibiao_95 WHERE CId='A001')as b

    ON a.Sid=b.Sid

    (4)对比,满足A002,未必满足A001方向结果

    命令:SELECT a.Sid,a.Cid,a.score,b.Cid,b.score from

    (SELECT * from ex_chengjibiao_95 WHERE Cid='A002')as a

    right join

    (SELECT * from ex_chengjibiao_95 WHERE CId='A001')as b

    ON a.Sid=b.Sid

    left join(左连接) ,满足左边条件,右边可能为空

    right join(右连接),满足右边条件,左边可能为空

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  • 原文地址:https://www.cnblogs.com/zgqbky/p/13425494.html
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