为什么在 MySQl 中有左连接和右连接? [英] why there is a left join and right join in MySQl?

问题描述

我认为左/右联接就足够了，如果交换表名，另一个将派生出来，有人知道为什么同时存在左联接和右联接吗?谢谢

I think either left/right join is sufficient, the other will derived if table names are interchanged, Anyone know why there is both left and right join? Thanks

推荐答案

join 求值的顺序一般是从左到右.所以 a join b join c 的执行就像你把它放在括号里 (a join b) join c 一样.

The order of join evaluation is generally left-to-right. So a join b join c executes as if you had parenthesized it (a join b) join c.

优化器可以对内连接重新排序，因为内连接是可交换的.重新排序外连接不是可交换的(即 a left join b 与 b left join a 不同)，因此优化器不能交换它们.

The optimizer can reorder inner joins because inner joins are commutative. Reordering outer joins is not commutative (i.e. a left join b is not the same as b left join a), so the optimizer can't swap them.

如果要对连接的一对表的结果进行左外连接，请使用括号，例如:a left join (b join c).

If you want to do a left outer join to the result of a joined pair of tables, use parentheses, like: a left join (b join c).

但是您也可以通过切换到右外连接来利用从左到右的求值顺序:b join c 右外连接 a -- 不带括号！

But you can also take advantage of the left-to-right order of evaluation by switching to a right outer join: b join c right outer join a -- without parentheses!

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