MySQL Update query with left join and group by

I take it that (Index1, Index2) is a unique key on Table, otherwise I would expect the reference to t.SpecialEventCount to result in an error.

Edited query to use subquery as it didn't work using GROUP BY

UPDATE
    Table AS t
    LEFT JOIN (
        SELECT
            Index1,
            Index2,
            COUNT(EventType) AS NumEvents
        FROM
            MEvents
        WHERE
            EventType = 'A' OR EventType = 'B'
        GROUP BY
            Index1,
            Index2
    ) AS m ON
        m.Index1 = t.Index1 AND
        m.Index2 = t.Index2
SET
    t.SpecialEventCount = m.NumEvents
WHERE
    t.SpecialEventCount IS NULL

my example

update card_crowd  as cardCrowd
LEFT JOIN 
(
select cc.id , count(1) as num
from card_crowd cc LEFT JOIN 
card_crowd_r ccr on cc.id = ccr.crowd_id
group by cc.id
) as tt
on cardCrowd.id = tt.id
set cardCrowd.join_num = tt.num;

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.