MySQL 添加联接后查询中断

Question

我有一个查询，它在我的 CMS 中循环加载数据以显示帖子。在我离开加入评论栏之前，投票栏中的一切都运行良好。评论显示正常，但总票数、赞成票和反对票数严重偏差。如果您需要查看表格，请告诉我。

SELECT
count(DISTINCT comment.comment ) AS Comment,
idea.dateofcreation AS timestamp, 
idea.userId AS userId,
idea.id AS ID,
idea.text AS Idea,
page.permalink AS Permalink,
user.name AS Username,
COUNT(CASE WHEN votelog.vote !="" THEN 1 END) AS 'totalvotes',
COUNT(CASE WHEN votelog.vote = '1' THEN 1 END) AS 'upvote',
COUNT(CASE WHEN votelog.vote = '-1' THEN 1 END) AS 'downvote'
FROM idea
LEFT JOIN votelog ON idea.id = votelog.ideaid
LEFT JOIN user ON idea.userId = user.id
LEFT JOIN page ON idea.id = page.ideaid
LEFT join comment ON comment.ideaid = idea.id 
GROUP BY idea.id
ORDER BY totalvotes DESC

Answer 1

您有不止一条评论。所以，在加入之前聚合：

SELECT c.num_comments,
       i.dateofcreation AS timestamp, i.userId AS userId, i.id AS ID, i.text AS Idea,
       p.permalink AS Permalink, p.name AS Username,
       SUM( vl.vote <> '' ) AS totalvotes,
       SUM( vl.vote = 1 ) AS upvote,
       SUM( vl.vote = -1 ) AS downvote
FROM idea i LEFT JOIN
     votelog vl
     ON i.id = vl.ideaid LEFT JOIN
     user u
     ON i.userId = u.id LEFT JOIN
     page p
     ON i.id = p.ideaid LEFT JOIN
     (SELECT c.ideaid, COUNT(*) as num_comments
      FROM comment c
      GROUP BY c.ideaid
     ) c
     ON c.ideaid = i.id 
GROUP BY c.num_comments,
       i.dateofcreation i.userId, i.id, i.text,
       p.permalink, p.name
ORDER BY totalvotes DESC;

备注：

Table 别名使查询更易于编写和阅读。
在 GROUP BY 中包含所有非聚合列是一个很好的做法（MySQL 的较新版本倾向于强制执行此操作）。
我假设选票是数字。将它们与数字进行比较，而不是与字符串进行比较。
MySQL 有一个很好的 shorthand 用于计算布尔表达式为真的次数。

Answer 2

看来您决定加入 table comments 只是为了计算有多少评论。

但是通过这样做，您现在会生成多行，每个评论一行，这会减少所有其他计数。

我建议使用标量子查询来计算评论数，并删除连接。类似于：

SELECT
(select count(DISTINCT comment) from comment c where c.ideaid = idea.id) AS Comment,
idea.dateofcreation AS timestamp, 
idea.userId AS userId,
idea.id AS ID,
idea.text AS Idea,
page.permalink AS Permalink,
user.name AS Username,
COUNT(CASE WHEN votelog.vote !="" THEN 1 END) AS 'totalvotes',
COUNT(CASE WHEN votelog.vote = '1' THEN 1 END) AS 'upvote',
COUNT(CASE WHEN votelog.vote = '-1' THEN 1 END) AS 'downvote'
FROM idea
LEFT JOIN votelog ON idea.id = votelog.ideaid
LEFT JOIN user ON idea.userId = user.id
LEFT JOIN page ON idea.id = page.ideaid
GROUP BY idea.id
ORDER BY totalvotes DESC

MySQL 添加联接后查询中断

MySQL query broke after adding join

mysql

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content-management-system