Created
November 14, 2017 06:48
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#交叉连接 cross join :没有 WHERE 子句的交叉联接将产生连接所涉及的表的笛卡尔积。第一个表的行数乘以第二个表的行数等于笛卡尔积结果集的大小。 | |
select * from student cross join course | |
#三张表:A (id,xxx) B(id,xxx) C(id,xxx) 要根据相同的id显示三张表,做Left Join。 | |
select * from a inner join b on a.id=b.id inner join c on b.id=c.id | |
SELECT XXX | |
FROM ((A LEFT JOIN B ON A.id = B.id) | |
LEFT JOIN C ON A.id = C.id) | |
WHERE B.id Is Not Null |
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