Mysql:如何获取评分数据

Question

我想获取每个提要及其相应的 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

但它只返回一行，而实际上我有 2 user_feeds 。如果我删除聚合函数 sum，它会返回两个 user_feed。

Answer 1

将group by与SUM结合使用

group by feed_id

尝试这个查询

 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

aggregate function performs a calculation on a set of values and returns a single value