优化 join on性能,解决联表查询慢的问题
这里只提供一种方式啊,就是如果连表有查询条件,那就先把条件查了,然后再连表,这个很有用
比如:pub_user1 与 pub_user2 有相同的字段 user_id
直接这么写会多查询很多数据
SELECT a.user_id, a.user_name, b.user_code, b.user_num FROM pub_user1 a RIGHT JOIN pub_user2 b ON a.user_id = b.user_id AND b.user_num > 0;
可以改成这样:
SELECT a.user_id, a.user_name, b.user_code, b.user_num FROM pub_user1 a RIGHT JOIN ( SELECT user_id, user_code, user_num FROM pub_user2 WHERE user_num > 0 ) AS b ON a.user_id = b.user_id;
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