MYSQL从其他表更新数据

Question

我有两个这样的结构表：

content (content_id, content_type, user_id, time, comment_count)

comments (comment_id, content_id, userid, comment, comment_time)

我想要做的是用评论总和更新comments_count字段，即评论表中的COUNT（content_id）。

我无法弄清楚正确的语法

Answer 1

UPDATE content c1 SET comment_count=(
    SELECT COUNT(c2.content_id) FROM comments c2 
    WHERE c1.content_id = c2.content_id
)

Answer 2

Cross Table Update with MySQLCross Table Update with MySQL

UPDATE product p, productPrice pp
SET pp.price = pp.price * 0.8
WHERE p.productId = pp.productId
AND p.dateCreated < '2004-01-01'