需要计算文件夹，mysql表中有多少文件

Question

我有两张桌子

documents: id, folder_id

folders: id, title

我想检索文件夹列表以及与该文件夹相关的文件数量（documents.folder_id = folders.id）

我试过这个

select 
 f.title, count(d.id) as file_count
from
 folders f
left join
 documents d on f.id = d.folder_id

但是只返回一行，文件表中的总行数

Answer 1

您缺少GROUP BY子句，以便按f.title返回一行。

select 
 f.title, count(d.id) as file_count
from
 folders f
left join
 documents d on f.id = d.folder_id
GROUP BY f.title

请注意，以上将省略空文件夹，我相信。如果您想为空文件夹计算零，请改为使用COUNT(d.*)：

select 
 f.title, count(d.*) as file_count
from
 folders f
left join
 documents d on f.id = d.folder_id
GROUP BY f.title

Answer 2

SELECT F.id, F.title, COUNT(D.*)
FROM folders AS F
     LEFT OUTER JOIN documents AS D ON D.folder_id = F.id
GROUP BY F.id;