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Here's something that's been bugging me with SQL recently. Is it possible to create a single query that that can create a list of one non-unique value which *never* has a certain corresponding value in another field? ok.. doesn't make sense... take this table:
and to derive the results
i.e a list of A which have NOT ever had a "yes" next to them. I know this is pretty easy to find using a subquery (i.e. find all the ones that HAVE had a yes and subtract that from a full list) and the equivalent join too, however I'm wondering if there's a way to do this with just ONE query, possibly using a small inline function to make any "yes"'s be selected as the result of an aggregate of some sort, and then lose those from a HAVING or such.....
SELECT fld1 from table1
SELECT fld1 from table1 where
This executes fast - the reason is that you MUST do a full-table scan to complete the query anyway. So both 'selects' actually use data from a single full table scan. Sub-queries are not necessarily evil. In this instance (using Oracle) the performance will be the same whether you use the above query or something more complex. You may even degrade performance by using something else.