MySQL Update query with left join and group by
- by Rob
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.
