MySQL语句组合连接和计数？ [英] MySQL statement combining a join and a count?

问题描述

我有一个文件夹表。我想返回userId为16的所有记录。

I've got a table of 'folders'. I want to return all the records with the userId of 16.

SELECT * FROM `folders` WHERE userId = 16;

我有一个'files'表。对于上面返回的每个文件夹，我想返回该文件夹中的文件计数。

I've got a table of 'files'. For each 'folder' returned above, I want to return a count of 'files' within that 'folder'.

SELECT COUNT(*) as "Files" FROM files WHERE Folder = n;

如何组合这些？我迷路了。谢谢！

How do I combine these? I'm lost. Thanks!

推荐答案

SELECT  fol.*
 ,      (       SELECT  COUNT(*)
                FROM    files           fil
                WHERE   fil.Folder      = fol.Folder
        )       AS      "Files"
FROM    folders         fol
WHERE   fol.userId      = 16

它称为相关子查询。

http://dev.mysql.com/doc/refman/5.1/en/ correl-subqueries.html

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