The subquery will always return a row from LogEvent, but that row's itemID
will be null if theitemID doesn't match a row from Item. That's why the subquery has the
"and i.ItemID is null".
> Stephan Szabo wrote:
>> On Thu, 26 Feb 2004 mike(at)linkify(dot)com wrote:
>>>I'm using postgresl 7.3.2 and have a query that executes very slowly.
>>>There are 2 tables: Item and LogEvent. ItemID (an int4) is the
>>>primary key of Item, and is also a field in LogEvent. Some ItemIDs in
>>>LogEvent do not correspond to ItemIDs in Item, and periodically we
>>>need to purge the non-matching ItemIDs from LogEvent.
>>>The query is:
>>>delete from LogEvent where EventType != 'i' and ItemID in
>>>(select distinct e.ItemID from LogEvent e left outer join Item i on
>>>e.ItemID = i.ItemID where e.EventType != 'i' and i.ItemID is null);
>>>I understand that using "in" is not very efficient.
>>>Is there some other way to write this query without the "in"?
>> delete from LogEvent where EventType != 'i' and not exists
>> (select * from Item i where i.ItemID=LogEvent.ItemID);
> Maybe I'm not reading his subquery correctly, but the left outer
> join will produce a row from LogEvent regardless of whether or not a
> matching row exists in Item, correct? So doesn't it reduce to:
> DELETE FROM LogEvent WHERE EventType <> 'i';
> Mike Mascari
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