Mysql：如何获取评级数据

Question

我希望每个Feed都有相应的rating。 我有这个mysql查询

    SELECT
    user_feed.*, COALESCE(SUM(feed_rating.rating), 0) AS rating
FROM
    user_feed
LEFT JOIN
    feed_rating
ON
    feed_rating.feed_id = user_feed.id

但实际上我只有2 user_feeds，它只返回一行。 如果我删除聚合函数sum，则返回两个user_feeds。

Answer 1

将group by与SUM

一起使用

group by feed_id

尝试此查询

 SELECT
    user_feed.*, COALESCE(SUM(feed_rating.rating), 0) AS rating
FROM
    user_feed
LEFT JOIN
    feed_rating
ON
    feed_rating.feed_id = user_feed.id group by feed_id

  

聚合函数对一组值执行计算   返回单个值