Question

I have a left join to a table and want to count columns from it, after grouping by a column of the parent table:

SELECT * , COUNT(list.id) AS listcount, COUNT(uploads.id) AS uploadcount 
FROM members 
LEFT JOIN lists ON members.id= list.mid
LEFT JOIN uploads ON members.id= uploads.mid
GROUP BY members.id

Assume that a user can have either lists or uploads based on the type of user. Then is above query good enough? If not why?

Or do I have to use this query?

SELECT * , l.listcount,   u.uploadcount 
FROM members 
LEFT JOIN (select count(lists.id) as listscount,mid from lists group by mid) as l
                                                                on l.mid = m.id
LEFT JOIN (select count(uploads.id) as uploadscount
                           ,mid from uploads group by mid) as u on u.mid = m.id
GROUP BY members.id

Or correlated subqueries?

SELECT *,
      (select count(lists.id) as listscount from lists as l where l.mid = m.id
       group by mid) as listcount
      (select count(uploads.id) from uploads as u where u.mid = m.id
       group by mid) as uploadscount 
FROM members 
GROUP BY members.id

And which is best solution?

Answer 1

The alias m for members is missing in query 2 and 3. Otherwise they should give the same numbers.

Query 2 (fixed) will perform fastest.

Query 1 is different in that it will give a higher number for uploads, if there are cases of multiple lists per member. After joining to lists, there will be multiple rows for a member too, which will increase the count for uploads. So query 1 is probably wrong.

Also, NULL values are not counted. The manual informs:

COUNT(expr)

Returns a count of the number of non-NULL values of expr in the rows
retrieved by a SELECT statement. The result is a BIGINT value.