mysql update query with sub query

Question

Can anyone see what is wrong with the below query?

When I run it I get:

#1064 - You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near 'a where a.CompetitionID = Competition.CompetitionID' at line 8

Update Competition Set Competition.NumberOfTeams = ( SELECT count(*) as NumberOfTeams FROM PicksPoints where UserCompetitionID is not NULL group by CompetitionID ) a where a.CompetitionID =  Competition.CompetitionID 

Answer 1

The main issue is that the inner query cannot be related to your where clause on the outer update statement, because the where filter applies first to the table being updated before the inner subquery even executes. The typical way to handle a situation like this is a multi-table update.

Update   Competition as C   inner join (     select CompetitionId, count(*) as NumberOfTeams     from PicksPoints as p     where UserCompetitionID is not NULL     group by CompetitionID   ) as A on C.CompetitionID = A.CompetitionID set C.NumberOfTeams = A.NumberOfTeams 

Demo: http://www.sqlfiddle.com/#!2/a74f3/1

Answer 2

Thanks, I didn't have the idea of an UPDATE with INNER JOIN.

In the original query, the mistake was to name the subquery, which must return a value and can't therefore be aliased.

UPDATE Competition SET Competition.NumberOfTeams = (SELECT count(*) -- no column alias   FROM PicksPoints   WHERE UserCompetitionID is not NULL   -- put the join condition INSIDE the subquery :   AND CompetitionID =  Competition.CompetitionID   group by CompetitionID ) -- no table alias 

should do the trick for every record of Competition.

To be noticed :

The effect is NOT EXACTLY the same as the query proposed by mellamokb, which won't update Competition records with no corresponding PickPoints.

Since SELECT id, COUNT(*) GROUP BY id will only count for existing values of ids,

whereas a SELECT COUNT(*) will always return a value, being 0 if no records are selected.

This may, or may not, be a problem for you.

0-aware version of mellamokb query would be :

Update Competition as C LEFT join (   select CompetitionId, count(*) as NumberOfTeams   from PicksPoints as p   where UserCompetitionID is not NULL   group by CompetitionID ) as A on C.CompetitionID = A.CompetitionID set C.NumberOfTeams = IFNULL(A.NumberOfTeams, 0) 

In other words, if no corresponding PickPoints are found, set Competition.NumberOfTeams to zero.