MySQL: COUNT() from multiple tables

Question

I have 3 tables user , person and company .
the user_id is a foreign key in both person and company tables.
what I want to do is to count how many persons and companies are related to each user, so I did the following query:

select u.id , u.username, count(c.user_id) as count_company, count(p.user_id) as count_people
from user u left join company c on (u.id = c.user_id)
left join person p on (u.id = p.user_id)
group by u.id, u.username

but I'm getting wrong result!!
This is the result I get:

however, when I try to count only the companies related to each user using this query:

select u.id , u.username, count(c.staff_user_id) as count_company
from fos_user u left join company c on (u.id = c.staff_user_id)
group by u.id, u.username

I get this result:

what am I missing here ?

Answer 1

If a user is related to multiple companies and/or multiple persons, then you always count the cross join of them - Which is then the number of companies multiplied with the number of persons.

To solve that problem (in your case) I would just use subqueries:

select u.id, u.username, 
    (select count(*) from company c where u.id = c.user_id) as count_company,
    (select count(*) from person p  where u.id = p.user_id) as count_people
from user u

Note that a double join like in your query is from relational point of view (in most cases) just wrong.

Answer 2

seems you need the distinct count

select u.id 
    , u.username
    , count(distinct c.user_id) as count_company
    , count(distinct p.user_id) as count_people
from user u 
left join company c on (u.id = c.user_id)
left join person p on (u.id = p.user_id)
group by u.id, u.username

count(col) count the not null values for col in relation ... count(distinct col) count the distinct value for col1