从 2 个表中选择行数并合并为一行（mysqli）

Question

我通过 php 和 mysqli 创建了一个像 facebook 这样的网络应用程序

在我的应用程序中，我有一个帖子表，一个点赞表和一个 cmets 表

我想用他的 post_id 获取一行中每个帖子的 cmets 和喜欢的数量！！！

我尝试了一些这样的查询：

select `tblpost`.`post_id`,  COALESCE(TCOMM.`comment_num`,0) as `c_num`, COALESCE(TLIKE.`like_num`,0) as `l_num`
from 
(select `tblpost`.`post_id`, count(*) as `like_num` from `tblpost` join `tbllikes` on `tbllikes`.`post_id` = `tblpost`.`post_id` group by `tblpost`.`post_id`
) TLIKE
inner join
(select `tblpost`.`post_id`, count(*) as `comment_num` from `tblpost` join `tblcomments` on `tblcomments`.`post_id` =  `tblpost`.`post_id` group by `tblpost`.`post_id`) TCOMM
on
    TCOMM.`post_id` = TLIKE.`post_id`

但我不知道我的问题是什么

Answer 1

您可以使用两个左连接来计算不同值。

如果tbllikes 和tblcomments 表中有字段like_id 和comment_id，这样的方法会起作用

SELECT 
    tblpost.post_id AS post_id, 
    COUNT(DISTINCT tbllikes.like_id) AS likes,
    COUNT(DiSTINCT tblcomments.comment_id) AS comments
FROM tblpost 
LEFT JOIN tbllikes ON tbllikes.post_id = tblpost.post_id
LEFT JOIN tblcomments on tblcomments.post_id =  tblpost.post_id
GROUP BY tblpost.post_id

Answer 2

首先，我认为您可以大大简化您的查询：

select l.post_id,
        COALESCE(c.comment_num, 0) as c_num, COALESCE(l.like_num, 0) as l_num
from  (select l.post_id, count(*) as like_num
       from tbllikes l 
       group by l.post_id
      ) l inner join
      (select c.post_id, count(*) as comment_num
       from tblcomments c
       group by c.post_id
      ) c
      on l.post_id = c.post_id;

这只会让您发布既有喜欢又有 cmets 的帖子。要获得您想要的，请使用left join：

select p.post_id,
      COALESCE(c.comment_num, 0) as c_num, COALESCE(l.like_num, 0) as l_num
from tblpost p left join
     (select l.post_id, count(*) as like_num
      from tbllikes l 
      group by l.post_id
     ) l
     on l.post_id = p.post_id left join
     (select c.post_id, count(*) as comment_num
      from tblcomments c
      group by c.post_id
     ) c
     on c.post_id = p.post_id;