从另一个表更新表

Question

我有一个表，用于在posts表中存储用户提交的帖子。用户还可以“喜欢这些帖子。相似内容存储在likes表格中，每行一行，列id，user和postid。

我现在已在likes表格中添加了一列posts。如何更新每个帖子的likes表格中的posts列，以反映likes表中该帖子的相似数量？

TIA！

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)

<强> 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);