MySql：如何进行子查询并计算两个表中ID相同的所有行

Question

How to make a query which return values for specific ID's not for all.

    SELECT content.id, count(case likes.type when 'p' then 1 else null end) as p
FROM content
JOIN likes on likes.content_id = content.id

this code returns:

ID p
1 18

but i want it:

ID p
1 12
2 4
3 2

Answer 1

Add a group by

SELECT content.id, 
       sum(likes.type = 'p') as p
FROM content
JOIN likes on likes.content_id = content.id
GROUP BY content.id

Then the aggregate functions (ie count() ) are applied to the groups and not to the whole result.

Answer 2

You query would fail in most databases. The problem is that content.id is not summarized in the select but you are using an aggregation function.

This is a simple problem to fix:

SELECT content.id, count(case likes.type when 'p' then 1 else null end) as p
FROM content
JOIN likes on likes.content_id = content.id
GROUP BY content.id;

However, in general, you should be careful and always include all non-aggregated columns in the select in a group by .