左连接,排序,MySQL优化

我有这样的查询：

   SELECT m...., a...., r....
     FROM 0_member AS m                 
LEFT JOIN 0_area AS a ON a.user_id = (SELECT user_id 
                                        FROM `0_area` 
                                       WHERE user_id = m.id 
                                    ORDER BY sec_id ASC LIMIT 1)
LEFT JOIN 0_rank as r ON a.rank_id = r.id 
    WHERE m.login_userid = '$username'

我们的想法是从0_area表中获取第一行,从而获得内部连接.但是,它没有按预期工作.

在0_area和0_member之间,0_member.id = 0_area.user_id.但是,有多行0_area.user_id,我希望行具有最低值sec_id.

有什么帮助吗？

解决方法:

   SELECT m...., a...., r....
     FROM 0_member AS m               
LEFT JOIN (SELECT user_id, min(sec_id) minsec
           FROM `0_area`
           GROUP BY user_id) g1 on g1.user_id=m.id
LEFT JOIN 0_area AS a ON a.user_id = g1.user_id and a.sec_id=minsec
LEFT JOIN 0_rank as r ON a.rank_id = r.id 
    WHERE m.login_userid = '$username'

标签：outer-join,sql,mysql,left-join
来源： https://codeday.me/bug/20190827/1737159.html