MySQL Update query with left join and group by

Question

I am trying to create an update query and making little progress in getting the right syntax. The following query is working:

SELECT t.Index1, t.Index2, COUN         


        

Answer 1

Doing a left join with a subquery will generate a giant temporary table in-memory that will have no indexes.

For updates, try avoiding joins and using correlated subqueries instead:

UPDATE
    Table AS t
SET
    t.SpecialEventCount = (
        SELECT COUNT(m.EventType)
        FROM MEvents m
        WHERE m.EventType in ('A','B')
          AND m.Index1 = t.Index1
          AND m.Index2 = t.Index2
    )
WHERE
    t.SpecialEventCount IS NULL

Do some profiling, but this can be significantly faster in some cases.