How to Fix \"Error #1064: You have an error in your SQL syntax\" in MySQL Update Query with Subquery?

Troubleshooting a MySQL Update Query with a Subquery

In a MySQL update query with a subquery, you may encounter an error similar to this:

#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

Query:

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

Issue:

The error arises because the where clause in the outer update statement (where a.CompetitionID = Competition.CompetitionID) cannot be linked to the inner subquery. The where filter is applied to the table being updated (Competition) before the inner subquery executes.

Resolution:

To resolve this issue, use 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

Explanation:

This query joins the Competition table (aliased as C) with a subquery (aliased as A) that calculates the number of teams for each competition. The join condition is on CompetitionID. Then, the update statement sets the NumberOfTeams column in the Competition table to the calculated value from the subquery.

Demo:

You can try this updated query on SQL Fiddle: http://www.sqlfiddle.com/#!2/a74f3/1

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