How do I use a MySQL subquery to count the number of rows in a foreign table?

Question

I have two tables, users and reports . Each user has no, one, or multiple reports associated with it, and the reports table has a user_id field.

I have the following query, and I need to add to each row a count of how many reports the user has:

SELECT *
FROM users
LIMIT 1, 10

Do I need to use a subquery, and if so, how can I use it efficently? The reports table has thousands and thousands of rows.

Answer 1

There's no need for a subquery:

SELECT users.user_id, COUNT(reports.user_id) AS number_of_reports
FROM users
LEFT JOIN reports ON users.userid = reports.userid
GROUP BY users.user_id

To make the query more efficient, make sure there's indexes on the user_id fields in both tables

comment followup: COUNT function does not count nulls, so it'll return 0 (as expected) for any users which have no reports at all (the join would return a NULL for reports.user_id). Also added the GROUP BY bit, forgot that the first time around.

Answer 2

SELECT users.userid,
       SUM( IF( Reports.userid > 0, 1, 0 )) as TotRpts
   FROM 
       users LEFT JOIN reports
          ON users.userid = reports.userid;
   GROUP BY
       users.userid

you might need to change the IF() to

  IF( Reports.UserID is null, 0, 1 )