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

It creates a list of triplets Index1,Index2,EventCounts. It only does this for case where t.SpecialEventCount is NULL. The update query I am trying to write should set this SpecialEventCount to that count, i.e. COUNT(m.EventType) in the query above. This number could be 0 or any positive number (hence the left join). Index1 and Index2 together are unique in Table t and they are used to identify events in MEvent.

How do I have to modify the select query to become an update query? I.e. something like

UPDATE Table SET SpecialEventCount=COUNT(m.EventType).....

but I am confused what to put where and have failed with numerous different guesses.