Difficult MySQL Query - mysql

Question

In our database tables we keep a number of counting columns to help reduce the number of complex look-up queries. For example, in our users table we have columns for the number of reviews written, photos uploaded, friends, followers, etc. To help make sure these stay in sync we have a script that runs periodically to check and update these counting columns.
I've been attempting to write an efficient query that calculates the number of friends a specific user has and haven't been able to figure out how to do it. In our friend model someone is a friend if they add you as a friend (no confirmation needed), so you have to count the number of unique of people when adding the number of friends you have added plus the number of people who have added you as a friend.
Here are two queries that each work to update the friend count for all the rows in our users table for friendship in a single direction. What I can't figure out it how to combine them so you get the total number of unique friends a user has:
Users Who Added You As A Friend
UPDATE users
JOIN (SELECT cid2, COUNT(*) as c
FROM connections
JOIN users ON connections.cid1 = users.user_id
WHERE connection_type = "MM"
AND connections.status="A"
AND users.status != "D"
GROUP BY cid2) f ON f.cid2 = users.user_id
SET users.friends = f.c
WHERE users.status != "D";
Users Who You Added As A Friend
UPDATE users u
JOIN (SELECT cid1, COUNT(*) as c
FROM connections
JOIN users ON connections.cid1 = users.user_id
WHERE connection_type = "MM"
AND connections.status = "A"
AND users.status != "D"
GROUP BY cid1) f ON f.cid1 = users.user_id
SET users.friends = f.c
WHERE users.status != "D";

Answer 1

Attempt 3...
UPDATE
users
INNER JOIN
(
SELECT
connections.cid1 AS user_id,
COUNT(*) AS total
FROM
(
SELECT cid1, cid2 FROM connections WHERE connection_type = 'MM' AND status = 'A'
UNION
SELECT cid2, cid1 FROM connections WHERE connection_type = 'MM' AND status = 'A'
)
AS connections
INNER JOIN
users
ON users.user_id = connections.cid2
AND users.status != 'D'
GROUP BY
connections.cid1
)
AS friends
ON friends.user_id = users.user_id
SET
users.friends = friends.total
WHERE
users.status != 'D';
(Other options deleted at OP's request. See edit history if interested.)

Answer 2

You could write it as a stored procedure which does both SELECT COUNT() and then adds it to a counter variable, then issue a single UPDATE

Answer 3

How important is the status flag? You're going to run into trouble trying to update the users table with a sub-query which references the same table.
Consider:
UPDATE connections SET status='D' WHERE cid1 IN
(SELECT user.id
FROM users u
WHERE status='D');
UPDATE connections SET status='D' WHERE cid2 IN
(SELECT user.id
FROM users u
WHERE status='D');
UPDATE users u
SET friends = (
SELECT COUNT(DISTINCT friend)
FROM (
SELECT c1.cid2 as friend
FROM connections c1
WHERE c1.connection_type = 'MM'
AND c1.connections.status='A'
AND c1.status<>'D'
AND c1.cid1=u.id
UNION
SELECT c2.cid1 AS friend
FROM connections c2
WHERE c2.connection_type = 'MM'
AND c2.connections.status='A'
AND c2.status<>'D'
AND c2.cid1=u.id
)
)