MySQL COUNT(*) – How to Use on Multiple Tables

mysqlsql

What's wrong with this query:

SELECT co.*, mod.COUNT(*) as moduleCount, vid.COUNT(*) as vidCount 
 FROM courses as co, modules as mod, videos as vid 
 WHERE mod.course_id=co.id AND vid.course_id=co.id ORDER BY co.id DESC

In other words, how can I do it so with every record returned from 'courses', there's
an additional column called 'modCount' which shows the number of records in the modules table for that course_id, and another called 'vidCount' which does the same thing for the videos table.

Error:

Error Number: 1064

You have an error in your SQL syntax;
check the manual that corresponds to
your MySQL server version for the
right syntax to use near ') as
moduleCount, vid.COUNT() as vidCount
FROM courses as co, ' at line 1

Best Answer

Using subselects you can do:

SELECT co.*, 
    (SELECT COUNT(*) FROM modules mod WHERE mod.course_id=co.id) AS moduleCount, 
    (SELECT COUNT(*) FROM videos vid WHERE vid.course_id=co.id) AS vidCount
FROM courses AS co
ORDER BY co.id DESC

But be carefull as this is an expensive query when courses has many rows.

EDIT: If your tables are quite large the following query should perform much better (in favor of being more complex to read and understand).

SELECT co.*, 
    COALESCE(mod.moduleCount,0) AS moduleCount,
    COALESCE(vid.vidCount,0) AS vidCount
FROM courses AS co
    LEFT JOIN (
            SELECT COUNT(*) AS moduleCount, course_id AS courseId 
            FROM modules
            GROUP BY course_id
        ) AS mod
        ON mod.courseId = co.id
    LEFT JOIN (
            SELECT COUNT(*) AS vidCount, course_id AS courseId 
            FROM videos
            GROUP BY course_id
        ) AS vid
        ON vid.courseId = co.id
ORDER BY co.id DESC
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