Question

I have the following table in PostgreSQL database:

CREATE TABLE maclist (
    username character varying,
    mac macaddr NOT NULL,
    CONSTRAINT maclist_user_mac_key UNIQUE (username, mac)
);

I need a way to check if the MAC address is assigned to the user or the user has no assigned MAC addresses at all. Basically I need a query that returns row if all conditions are true or no condition is true ie NOT a XOR b.

EDIT:
Example:

username | mac
john     | 11:22:33:44:55:66
john     | 11:22:33:44:55:67
doe      | 11:22:33:44:55:68

If I query:

username = john, mac = 11:22:33:44:55:66                     -> true, 1 whatever...
username = john, mac != 11:22:33:44:55:66                    -> 0, null or nothing...
username = jane, mac = no matter what except john's or doe's -> true, 1 whatever...
username = jane, mac = john's or doe's                       ->  0, null or nothing...

I need true under two conditions:

1. There is a row for that (user, mac) combination. There are no rows
2. for that user AND there are no rows for that mac

So far I got this:

SELECT yes
FROM
  (SELECT 1 AS yes) AS dummy
LEFT JOIN maclist ON (username = 'user'
                      OR mac = '11:22:33:44:55:66')
WHERE ((username = 'user'
        AND mac = '11:22:33:44:55:66')
       OR (username IS NULL
           AND mac IS NULL);

It works, but it seems to me like a hack and I also have no idea about the performance of this query as the database grows.
My question is if there are any better ways to do this.

Answer 1

EDIT: I’ve revised the logic slightly, I think it matches what you want

The following code will return 1 under two conditions…

• There is a row for that user and that mac
• There are 0 rows for that user and there are 0 rows for that mac

Under all other conditions, the query returns 0.

• There are rows for that user but none of them are for that mac
• There are rows for that mac but none of them are for that user

SELECT
  CASE WHEN COUNT(*) = 0                                THEN 1
       WHEN SUM(CASE WHEN mac = '11:22:33:44:55:66'
                      AND user = 'user' THEN 1
                                        ELSE 0 END) = 1 THEN 1
                                                        ELSE 0
  END
FROM
  maclist
WHERE
     username = 'user'
  OR mac      = '11:22:33:44:55:66'