sql - Mysql COUNT(*) 在多个表上

Question

这个查询有什么问题:

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

换句话说，我该如何处理从“类(class)”返回的每条记录，有 一个名为“modCount”的附加列显示模块表中该 course_id 的记录数，另一个名为“vidCount”的列对视频表执行相同的操作。

错误:

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

Answer 1

使用子选择你可以:

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

但要小心，因为当类(class)有很多行时，这是一个昂贵的查询。

编辑: 如果您的表非常大，则以下查询应该执行得更好(有利于阅读和理解更复杂)。

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