Mysql COUNT(*) on multiple tables

Question

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         


        

Answer 1

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

Answer 2

SELECT co.*, m.ModCnt as moduleCount, v.VidCnt as vidCount 
FROM courses co
INNER JOIN (
        select count(*) AS ModCnt, co.id AS CoID
        from modules 
        group by co) m
    ON m.CoID = co.id
INNER JOIN (
        select count(*) AS VidCnt, co.id AS CoID
        from videos
        group by co) v
    ON v.CoID = co.id   
INNER JOIN videos vid 
    ON vid.course_id = co.id 
ORDER BY co.id DESC

Answer 3

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

Answer 4

i have better solution and easy

SELECT COUNT(*),(SELECT COUNT(*) FROM table2) FROM table1

Answer 5

Shoot this. I did the job with some non-mysql code:

function getAllWithStats($info='*',$order='',$id=0)
{
    $courses=$this->getAll($info,$order,$id);

    foreach ($courses as $k=>$v)
    {
        $courses[$k]['modCount']=$this->getModuleCount($v['id']);
        $courses[$k]['vidCount']=$this->getVideoCount($v['id']);
    }

    return $courses;
}