添加连接后 MySQL 查询中断

Question

我有一个查询，它为我的 CMS 中的循环加载数据以显示帖子。在我离开评论栏之前，投票栏中的一切都运行良好。评论显示正常，但总票数、赞成票和反对票数都大打折扣。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

看来你决定加入表comments只是为了计算有多少 cmets。

但是通过这样做，您现在会生成多行，每条评论一行，这会消除所有其他计数。

我建议使用标量子查询来计算 cmets，并删除连接。比如：

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

Answer 2

您有多个评论。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;

注意事项：

• 表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种很好的做法（并且最新版本的 MySQL 倾向于强制执行此操作）。
• 我假设选票是数字。将它们与数字而非字符串进行比较。
• MySQL 有一个很好的简写方式来计算布尔表达式为真的次数。