MySQL 总和,用 group by 和 join 计数
我有三种表格类型,帖子和见解.
I have three tables types, post and insights.
- 类型表包含帖子的类型.
- post 表包含已发布的帖子.
- 洞察力表包含每日帖子的洞察力.
这是我的 sql fiddle SQL Fiddle 的链接.
Here is the link to my sql fiddle SQL Fiddle.
现在我想生成一个报告,其中包含针对每种类型的帖子数量以及他们喜欢和评论的总和,即类型 |COUNT(post_id) |SUM(喜欢) |SUM(评论).
Now i want to generate a report which contains number of post against each type and the sum of their likes and comments i.e. Type | COUNT(post_id) | SUM(likes) | SUM(comments).
这些是我的尝试:
select type_name, count(p.post_id), sum(likes), sum(comments)
from types t
left join posts p on t.type_id = p.post_type
left join insights i on p.post_id = i.post_id
group by type_name;
结果:聚合值不正确.
select type_name, count(p.post_id), p.post_id,
(select sum(likes) from insights where post_id = p.post_id) as likes,
(select sum(comments)from insights where post_id = p.post_id) as comments
from types t
left join posts p on t.type_id = p.post_type
group by type_name;
结果:仅显示一个帖子的点赞和评论总数.
Result: Displays the sum of likes and comments of only one post.
推荐答案
您的第一次尝试非常接近.但是每个 post_id
都乘以 insights
中的匹配数,因此您需要使用 DISTINCT
:
Your first attempt was real close. But each post_id
was being multiplied by the number of matches in insights
, so you need to use DISTINCT
:
select type_name, count(distinct p.post_id), sum(likes), sum(comments)
from types t
left join posts p on t.type_id = p.post_type
left join insights i on p.post_id = i.post_id
group by type_name;
或者,您可以使用组合了同一帖子的所有见解的子查询进行分组:
Alternatively, you can group with a subquery that combines all the insights for the same post:
select type_name, count(*), sum(likes), sum(comments)
from types t
left join posts p on t.type_id = p.post_type
left join (select post_id, sum(likes) likes, sum(comments) comments
from insights
group by post_id) i on p.post_id = i.post_id
group by type_name;
小提琴
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