mysql - sql sum 使用一对多关系

Question

我有两张桌子

comments
------------------
id       text

rates
---------------------
id    comment_id    score

我的查询是……但求和运算不受影响。只有一排将被退回。

SELECT *, SUM(rates.score) FROM comment LEFT JOIN rate ON (rate.comment_id = comment.id)

Answer 1

SELECT comment.*, SUM(rates.score) FROM comment LEFT JOIN rate ON (rate.comment_id = comment.id)
GROUP BY comment.id

Answer 2

您当前的查询将简单地将所有评论的评级相加。您必须做的是合并GROUP BY，它将按每个评论对聚合进行分组，因此结果SUM()将只是每个特定评论的评分总和：

SELECT a.*, SUM(b.score) AS ratingsum
FROM comment a
LEFT JOIN rate b ON a.id = b.comment_id
GROUP BY a.id

Answer 3

尝试添加一个 group by 子句：

 SELECT comment.*, SUM(rates.score) FROM comment LEFT JOIN rate ON rate.comment_id = comment.id GROUP BY rates.comment_id;