MySQL sum, count with group by and joins

问题

I have three tables types, post and insights.

Types table contains the types of post.
post table contains the post that have been made.
the insight table contains the insights of post on daily basis.

Here is the link to my sql fiddle SQL Fiddle.

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).

These are my tries:

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;

Result: Aggregate values are not correct.

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;

Result: Displays the sum of likes and comments of only one post.

回答1:

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;

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;

FIDDLE

回答2:

try this

SELECT types.type_name, stats.posts, stats.likes, stats.comments
FROM types
LEFT JOIN (
select post_type, count(i.post_id) as posts, sum(i.likes) as likes, sum(i.comments) as comments
from insights i INNER JOIN posts p ON i.post_id = p.post_id
) as stats
ON types.type_id = stats.post_type;

来源：https://stackoverflow.com/questions/20756948/mysql-sum-count-with-group-by-and-joins

标签

mysql

select

join

group-by

aggregate-functions