加入与子查询

2021-11-20 00:00:00 join sql subquery mysql

我是一个老派的 MySQL 用户并且总是喜欢 JOIN 而不是子查询.但是现在每个人都使用子查询，我讨厌它；我不知道为什么.

I am an old-school MySQL user and have always preferred JOIN over sub-query. But nowadays everyone uses sub-query, and I hate it; I don't know why.

我缺乏理论知识来判断是否有任何差异.子查询是否与 JOIN 一样好，因此无需担心?

I lack the theoretical knowledge to judge for myself if there is any difference. Is a sub-query as good as a JOIN and therefore is there nothing to worry about?

推荐答案

摘自 MySQL 手册 (13.2.10.11 将子查询重写为连接):

LEFT [OUTER] JOIN 比等效的子查询更快，因为服务器可能能够更好地优化它——这一事实并不仅仅针对 MySQL 服务器.

A LEFT [OUTER] JOIN can be faster than an equivalent subquery because the server might be able to optimize it better—a fact that is not specific to MySQL Server alone.

因此子查询可能比 LEFT [OUTER] JOIN 慢，但在我看来，它们的优势在于可读性稍高.

So subqueries can be slower than LEFT [OUTER] JOIN, but in my opinion their strength is slightly higher readability.

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