Question

我有以下两个表格：用户：

         id 7dexpn
=========== ==========
          1 0
          2 0
          3 0


user_pages:
        id     user_id 7dexpf
========== =========== ==========
       99           1 0
       98           2 1
       97           3 1
       96           3 1
       95           3 1
       94           2 0

我已成功将{user_pages聚合的（7dexpf）标志}插入到user_pages（user_id）与users表（id）匹配的users表（7dexpn）中使用此查询

update users u join
       (select user_id, count(*) as cnt
        from user_pages 
        where `7dexpf` = 1
        group by user_id
       ) uu
       on uu.user_id = u.id
    set u.`7dexpn` = uu.cnt;

但是如果u。7dexpn = 0

，查询不会将标志更新回零

Answer 1

LEFT JOIN与COALESCE()：

一起使用

UPDATE users u 
LEFT JOIN
     (SELECT user_id, COUNT(*) as cnt
      FROM user_pages 
      WHERE `7dexpf` = 1
      GROUP BY user_id
     ) uu
 ON uu.user_id = u.id
SET u.`7dexpn` = COALESCE(uu.cnt,0);

通过id匹配条件，根据另一个更新一个MySQL表值

1 个答案: