当与LEFT JOIN，WHERE和OR结合使用时，MySQL JOIN返回不相关的行

Question

I have the following table structure. The idea is that users have permissions to a forum either by their class or specific user overrides. ('action' in both cases is an enum with values 'read' & 'write')

user (id, class)
forum (id, name)
forum_permissions (forum_id, class_id, action)
forum_user_permissions (forum_id, user_id, action)

With the following query, I'm getting extra results based on rows in forum_permissions that I don't expect. By this I mean that every row on forum_permissions with forum_id = 3 is returned even though the class_id does not match.

SELECT      forum.id AS forum_id, forum.name
FROM        forum
JOIN        forum_permissions ON forum_permissions.forum_id = forum.id
LEFT JOIN   forum_user_permissions ON ( 
             forum_user_permissions.forum_id = forum.id AND forum_user_permissions.user_id = 3 )
WHERE       (( forum_permissions.class_id = 1 AND forum_permissions.action = 'read' )
              OR
             ( forum_user_permissions.action = 'read' ))

eg I get this:

FORUM_ID    NAME
   1        chat
   2        support
   3        secret
   3        secret
   3        secret
   3        secret

but expected this:

FORUM_ID    NAME
   1        chat
   2        support
   3        secret

I have made an SQL Fiddle with the specific example including data http://sqlfiddle.com/#!2/75c3a/5/0

Answer 1

Your left join is adding those extra lines. mybe if you change the WHERE

WHERE  forum_user_permissions.user_id is not null and (
  (forum_permissions.class_id = 1 AND forum_permissions.action = 'read')
OR
  (forum_user_permissions.action = 'read')
)

Or

SELECT
 forum.id AS forum_id, forum.name
FROM
 forum
JOIN
 forum_permissions 
  ON
    forum_permissions.forum_id = forum.id
LEFT JOIN
 forum_user_permissions
  ON (
    forum_user_permissions.forum_id = forum.id 
  )
WHERE forum_user_permissions.user_id = 3 and (
 (forum_permissions.class_id = 1 AND forum_permissions.action = 'read')
 OR
 (forum_user_permissions.action = 'read')
 )

But it depends on the results your are trying to get

Answer 2

Okay, I solved it myself by using LEFT JOIN's for both permission tables, rather than put the logic in a WHERE clause. I'm not very sure if this is a better approach than my first attempt and will gladly upvote if someone can explain.

SELECT    forum.id AS forum_id, forum.name
FROM      forum
LEFT JOIN forum_permissions
        ON ( forum_permissions.forum_id = forum.id
        AND forum_permissions.class_id = 1
        AND forum_permissions.action = 'read' )
LEFT JOIN forum_user_permissions
       ON ( forum_user_permissions.forum_id = forum.id 
       AND forum_user_permissions.user_id = 3
       AND forum_user_permissions.action = 'read' )
WHERE forum_permissions.forum_id IS NOT null OR forum_user_permissions.forum_id IS NOT null

The full dataset is included below as I guess the fiddle will expire at some point.

INSERT INTO user
    (`id`, `class`)
VALUES
    (1, 1), (2, 1), (3, 1), (4, 2);

INSERT INTO forum
    (`id`, `name`)
VALUES
    (1, 'chat'), (2, 'support'), (3, 'secret'); 

INSERT INTO forum_permissions
    (`forum_id`, `class_id`, `action`)
VALUES
    (1, 1, 'read'), (1, 1, 'write'),
    (1, 2, 'read'), (1, 2, 'write'),
    (2, 1, 'read'), (2, 1, 'write'),
    (2, 2, 'read'), (2, 2, 'write'),
    (3, 2, 'read'), (3, 2, 'write'),
    (3, 3, 'read'), (3, 3, 'write');

INSERT INTO forum_user_permissions
    (`forum_id`, `user_id`, `action`)
VALUES
    (3, 3, 'read'), (3, 3, 'write');