SQL JOIN - 对所有左侧行进行分组

Question

所以，我有名为 files 和 folders 的表，它们具有 ID 和它们之间的通常关系：file.folder_id = folder.id

此外，files/folders 中的一些可以被字段 ignore 忽略。

我正在尝试获取文件夹列表和相应文件夹中的文件数。我的第一种方法运行良好但遗漏了空文件夹：

SELECT folders.id, folders.name, count(files.id) kount
FROM folders, files

WHERE folders.site_id=111
AND files.ignored=0
AND folders.ignored=0 
AND files.site_id=111
AND files.folder_id=folders.id

GROUP BY folders.name 
ORDER BY folders.name

所以我寻找 LEFT JOIN

SELECT folders.id, folders.name, count(files.id) kount
FROM folders 


LEFT JOIN files 
ON files.folder_id=folders.id

WHERE folders.site_id=111
AND folders.ignored=0
AND files.ignored=0
AND files.site_id=111


GROUP BY folders.name 
ORDER BY folders.name

但同样 - 缺少空文件夹。我做错了什么？

Answer 1

需要把条件过滤加入的table直接放到left join

SELECT folders.id, folders.name, count(files.id) kount
FROM folders 
LEFT JOIN files ON files.folder_id=folders.id
               AND files.ignored=0
               AND files.site_id=111
WHERE folders.site_id=111
AND folders.ignored=0
GROUP BY folders.name 
ORDER BY folders.name

Answer 2

试试这个。

SELECT folders.id, folders.name, count(files.id) kount
FROM folders 


LEFT JOIN files 
ON files.folder_id=folders.id
AND files.ignored=0
AND files.site_id=111

WHERE folders.site_id=111
AND folders.ignored=0



GROUP BY folders.name 
ORDER BY folders.name

SQL JOIN - 对所有左侧行进行分组

SQL JOIN - taking all left rows with grouping

sql

join

relation