Question

So I am in the process of optimising a SQL query. I have cleaned it up considerably but still have some work to do. See query below:

Competitions tbl:
ID, fb_share_points, tw_share_points

comp_mapper tbl:
id, referrer_user_id, user_subscription_id, has_fb_shared, has_tw_shared, competition_id

SELECT 
    IF (cm.has_fb_shared = 1, IFNULL(c.fb_share_points,0), 0) 
    + IF (cm.has_tw_shared = 1, IFNULL(c.tw_share_points,0), 0) 
    + (SELECT count(*) FROM comp_mapper as cm2
        WHERE cm2.comp_id = cm.comp_id 
        AND cm2.referrer_user_id = us.user_id)
    as shares
FROM competitions AS c
JOIN comp_mapper as cm ON cm.competition_id = c.id
JOIN user_subscription as us on cm.user_subscription_id = us.id
WHERE c.id = :id
ORDER BY shares DESC
LIMIT :limit

When a user is referred, the ID of the referring user is placed into the referrer_user_id column of the competition entrant. The subquery calculates how many competition entrants have referred other users. I am currently trying to find another way of writing this without the subquery because the query itself takes quite a while to run. If anyone has any suggestions or recommendations then please do share!

Answer 1

This should return an equivalent result. The difference here is that rather than a correlated subquery, we’ve got an inline view (MySQL calls it a derived table) which is aliased as rf.

SELECT IF(cm.has_fb_shared = 1, IFNULL(c.fb_share_points,0), 0) 
     + IF(cm.has_tw_shared = 1, IFNULL(c.tw_share_points,0), 0) 
     + IFNULL(rf.ref_count,0) AS shares
  FROM competitions c
  JOIN comp_mapper cm 
    ON cm.competition_id = c.id
  JOIN user_subscription us
    ON us.id = cm.user_subscription_id
  LEFT
  JOIN (  
         SELECT cm2.referrer_user_id 
              , COUNT(1) AS ref_count
           FROM comp_mapper cm2
          WHERE cm2.comp_id = :id
          GROUP BY cm2.referrer_user_id
       ) rf
    ON rf.referrer_user_id = us.user_id
 WHERE c.id = :id
 ORDER BY shares DESC
 LIMIT :limit

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It strikes me a bit odd that the query is returning a set of rows with only a single column (shares), and no other columns. If the intent is to return a single value, there would need to be an aggregate function (i.e. SUM() ) in the select list.