Question

How would I get the SUM of the mutual field here?

SELECT first_user_id, mutual
    FROM userprofile_usercontact
UNION ALL
SELECT second_user_id, mutual
    FROM userprofile_usercontact
GROUP BY first_user_id

In other words I want to get SUM(mutual) for (first_user_id + second_user_id) where mutual = 1.

Answer 1

SELECT u.user_id, SUM(u.mutual)
  FROM (SELECT first_user_id AS user_id, mutual
          FROM userprofile_usercontact
        UNION ALL
        SELECT second_user_id AS user_id, mutual
          FROM userprofile_usercontact
       ) AS u
 GROUP BY user_id;

The question in the comment asks:

How would I get rid of entries where mutual = 0?

Using a filter condition, as usual:

SELECT u.user_id, SUM(u.mutual)
  FROM (SELECT first_user_id AS user_id, mutual
          FROM userprofile_usercontact
        UNION ALL
        SELECT second_user_id AS user_id, mutual
          FROM userprofile_usercontact
       ) AS u
 WHERE u.mutual != 0   -- or u.mutual <> 0
 GROUP BY user_id;

The optimizer might well push the condition down into the UNION queries, so it would execute as if the query was written:

SELECT u.user_id, SUM(u.mutual)
  FROM (SELECT first_user_id AS user_id, mutual
          FROM userprofile_usercontact
         WHERE mutual != 0
        UNION ALL
        SELECT second_user_id AS user_id, mutual
          FROM userprofile_usercontact
         WHERE mutual != 0
       ) AS u
 GROUP BY user_id;

The net result is, of course, the same.