Question

I have these tables:

‘Users’

id  email                name                 last_access
1   [e-mail]       Luca Pluto           2012-10-05 17:21:22.0
2   [e-mail]      Irene Pippo          2012-10-05 17:22:25.0

‘Nets_permissions’

user_id         network_id  perm
1               1234        3
1               1235        1
2               1235        3

I’ve written this query:

SELECT u.id, u.name, n.perm
FROM users as u LEFT OUTER JOIN nets_permissions as n
ON u.id = n.user_id
WHERE n.network_id=1234 AND n.perm <> 3

because I want users that already have a permission for network_id (1234) , but also users that don’t have a permission for the network_id. In other words I would have this result query:

2    null    null

because the user Luca Pluto with id=1, for the net 1234, have perm=3 so I want left out. Instead, the user Irene Pippo with id=2 doesn’t have any permission on the 1234 net. So it’s row must have net_id and perm set to null.

My query result is empty. I don’t know why. Without the clause n.perm <> 3 seems to work well, but after also the null value are left out, not only the rws with perm=3.

I’ve also tried in this way:

SELECT u.id, u.name, n.perm FROM users as u LEFT OUTER JOIN 
(select * from nets_permissions WHERE network_id=1234) as n on u.id = n.user_id 
WHERE n.perm <> 3

but it doesn’t work. without the WHERE clause all works. After no. The result query is empty.

How I can resolve this problem? I need that the perm column is a value or null, I can’t remove this column.

Answer 1

The right solution is:

SELECT u.id, u.name, n.perm FROM users as u LEFT OUTER JOIN 
(select * from nets_permissions WHERE network_id=1234) as n on u.id = n.user_id 
WHERE (n.perm <> 3 or n.perm is NULL)

Here Working with NULL Values there is a expanation of the treatment of NULL values in MySQL

Thank you all for your help!