Question

I have the following tables:

tweets               retweets
-----------------    ----------------
user_id  retweets    user_id (etc...)
-----------------    ----------------
1        0           1
2        0           1
                     1
                     2
                     2

I want to count the number of retweets per user and update tweets.retweets accordingly:

UPDATE users 
SET retweets = (
  SELECT COUNT(*) FROM retweets WHERE retweets.user_id = users.user_id
)

I have been running this query two times, but it times out (on tables that are not that large). Is my query wring?

Also see the SQL Fiddle (although it apparently doesn’t allow UPDATE statements): http://www.sqlfiddle.com/#!2/f591e/1

Answer 1

This solution should be much faster than using subqueries for getting the count of tweets of each user (your correlated subquery will execute for each user):

UPDATE users a
LEFT JOIN
(
    SELECT user_id, COUNT(1) AS retweet_count
    FROM retweets
    GROUP BY user_id
) b ON a.user_id = b.user_id
SET a.retweets = COALESCE(b.retweet_count, 0)