Question

我正在尝试使用Posts和Comments的关联号码选择Likes。

这是我的查询

SELECT `waller_posts`.*,
       COUNT(waller_comments.id) AS num_comments,
       COUNT(waller_likes.id) AS num_likes
FROM `waller_posts`
LEFT JOIN `waller_comments` ON `waller_comments`.`post_id` = `waller_posts`.`id`
LEFT JOIN `waller_likes` ON `waller_likes`.`post_id` = `waller_posts`.`id`
WHERE `wall_id` = 1
  AND `wall_type` = "User"
GROUP BY `waller_posts`.`id`

当我在left join的这种情况下添加第二个likes时，num_comments和num_likes的结果出错了。我该如何进行这种查询？

Answer 1

该查询构建为在帖子上为您提供评论和喜欢的所有可能组合。

可能最容易使用COUNT（DISTINCT ...）： -

SELECT `waller_posts`.*,
       COUNT(DISTINCT waller_comments.id) AS num_comments,
       COUNT(DISTINCT waller_likes.id) AS num_likes
FROM `waller_posts`
LEFT JOIN `waller_comments` ON `waller_comments`.`post_id` = `waller_posts`.`id`
LEFT JOIN `waller_likes` ON `waller_likes`.`post_id` = `waller_posts`.`id`
WHERE `wall_id` = 1
  AND `wall_type` = "User"
GROUP BY `waller_posts`.`id`

请注意，您的查询依赖于MySQL的功能，但这会导致大多数SQL的错误。对于大多数SQL，您需要在GROUP BY子句中列出所有非聚合列。

Answer 2

使用Distinct子句，因为它将显示like和comment表数据的组合

SELECT `waller_posts`.*,
       COUNT(DISTINCT waller_comments.id) AS num_comments,
       COUNT(DISTINCT waller_likes.id) AS num_likes
FROM `waller_posts`
LEFT JOIN `waller_comments` ON `waller_comments`.`post_id` = `waller_posts`.`id`
LEFT JOIN `waller_likes` ON `waller_likes`.`post_id` = `waller_posts`.`id`
WHERE `wall_id` = 1
  AND `wall_type` = "User"
GROUP BY `waller_posts`.`id`

使用左连接计算多个表中的关联行

2 个答案: