Sunday, May 15, 2011

What's the different between these 2 sql's -- a condition to put into WHERE or ON

Q: What's the different between these 2 queries?

left outer join CodeLegend c on spv.StatusUid=c.CodeLegendUid
where c.FullName='Active'
------------------------------------------------------------------------ ----------
left outer join CodeLegend c on spv.StatusUid=c.CodeLegendUid
and c.FullName='Active'

%%A: all rows from outer table will show up in intermediate table. The "Active" condition becomes "and (c.FullName='Active' or c.FullName is null)" if put into ON clause

If in doubt, I usually put conditions in WHERE (not ON) because WHERE is less implicit and more explicit. Intermediate table tends to show more meaningful data this way. Not sure about this case though.

See http://searchoracle.techtarget.com/answer/LEFT-OUTER-JOIN-with-ON-condition-or-WHERE-condition

http://blog.sqlauthority.com/2009/03/15/sql-server-interesting-observation-of-on-clause-on-left-join-how-on-clause-effects-resultset-in-left-join/ explains right-table predicate in ON-clause

The more obscure issue is Outer table predicate in ON-clause, explained in other blog post...

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