mysql - 从其他表更新表

Question

我有一个表格，将用户提交的帖子存储在posts表格中。用户还可以“点赞”这些帖子。点赞存储在likes表格中，每个点赞一行，包含 、id和user列postid。

我现在在表格中添加了一likes列posts。如何为每个帖子更新表格中的likes列posts，以反映likes表格中该帖子的喜欢数量？

蒂亚！

Answer 1

UPDATE posts p
JOIN (SELECT postid, COUNT(*) c
      FROM likes
      GROUP BY postid) l
ON p.id = l.postid
SET p.likes = l.c

Answer 2

带有子查询的版本

UPDATE posts p
   SET likes = (SELECT COUNT(*) FROM likes WHERE postid = p.postid)

SQLFiddle

Answer 3

update posts p
left join
(
select postid, count(*) likes
from likes
group by postid
) l on p.id = l.postid
set p.likes = ifnull(l.likes, 0);