Question

I am coding a friendship system and it has two tables.

members

• id
• username
• password

friends

• id
• user_id
• friend_id
• status

Let’s say that i want a query that can select the friends IDs of the member $userId how possible to make this in one query?

I found a solution which is to make 2 queries. The fist selects the friends WHERE user_id = $userId AND the second selects friends WHERE friend_id = $userId and then MIX them in one array. If there is no other solution I’m going to use it.

please any ideas for both the SQL structure & Queries?

Answer 1

Use:

SELECT f.friend_id
  FROM FRIENDS f
 WHERE f.user_id = $user_id
UNION
SELECT t.user_id
  FROM FRIENDS t
 WHERE t.friend_id = $user_id

Using UNION will remove duplicates. UNION ALL would be faster, but it doesn’t remove duplicates.

If you want to get the information for the friends from the MEMBERS table, use:

SELECT m.*
  FROM MEMBERS m
  JOIN (SELECT f.friend_id 'user_id'
          FROM FRIENDS f
         WHERE f.user_id = $user_id
        UNION
        SELECT t.user_id
          FROM FRIENDS t
         WHERE t.friend_id = $user_id) x ON x.user_id = m.id

BTW: I hope you’re using mysql_escape_string on the variables, otherwise you risk SQL injection attacks: