Question

我的数据库中有四个不同的表：

主题

的thread_id
thread_content
时间戳

thread_rating：

thread_rating_id
的thread_id
喜欢
不喜欢

thread_report：

thread_report_id
的thread_id

thread_impression：

thread_impression_id
的thread_id

我将使用此SQL-Query

加入这些表

SELECT t.thread_id,
t.thread_content,
SUM(tra.liked) AS liked,
SUM(tra.disliked) AS disliked,
t.timestamp,
((100*(tra.liked + SUM(tra.liked))) / (tra.liked + SUM(tra.liked) + (tra.disliked + SUM(tra.disliked)))) AS liked_percent,
((100*(COUNT(DISTINCT tre.thread_report_id)) / ((COUNT(DISTINCT ti.thread_impression_id))))) AS reported_percent
FROM thread AS t
LEFT JOIN thread_rating AS tra ON t.thread_id = tra.thread_id
LEFT JOIN thread_report AS tre ON tra.thread_id = tre.thread_id
LEFT JOIN thread_impression AS ti ON tre.thread_id = ti.thread_id
GROUP BY t.thread_id
ORDER BY liked_percent

Query应该返回所有带有计算的喜欢和不喜欢的thread_ids，百分比的喜欢，时间戳，线程插入数据库的时间以及百分比的报告（时间，线程显示给用户）...

几乎所有的结果都是正确的，唯一不合适的结果就是喜欢和不喜欢。

如果我在查询前加上一个计数（*），我可以看到，正确的结果计数为1，错误的结果有时计数最多为60。好像有交叉连接问题...

我认为这是分组的问题，或者我应该接受联接。

我见过带有子选择的解决方案。但我认为这不是解决这个问题的好方法......

我在这里做错了什么？

Answer 1

tra表每个thread_id有多个记录。这导致SUM函数中出现双重计数在子选择中进行求和，按连接字段分组这样，您只需要tra2中的一个thread_id加入，并且将避免重复行。

SELECT t.thread_id,
  t.thread_content,
  tra2.liked
  tra2.disliked,
  t.timestamp,
  tra2.liked_percent,
  ((100*(COUNT(DISTINCT tre.thread_report_id)) / ((COUNT(DISTINCT ti.thread_impression_id))))) AS reported_percent
FROM thread AS t
LEFT JOIN (
     SELECT 
       tra.thread_id 
       , SUM(tra.liked) AS liked
       , SUM(tra.disliked) AS disliked
       , ((100*(tra.liked + SUM(tra.liked))) / (tra.liked + SUM(tra.liked) + (tra.disliked + SUM(tra.disliked)))) AS liked_percent 
     FROM thread_rating AS tra
     GROUP BY tra.thread_id
) as tra2 ON t.thread_id = tra2.thread_id
LEFT JOIN thread_report AS tre ON tra.thread_id = tre.thread_id
LEFT JOIN thread_impression AS ti ON tre.thread_id = ti.thread_id
GROUP BY t.thread_id
ORDER BY liked_percent DESC

左连接 - ＆gt;聚合函数问题

1 个答案: