Question

I have a table with a primary key ID column A, and several other data columns. I need to build a query that selects this column A when a unique combination of columns B & C, has more than 1 distinct value in column D. Thanks!

Answer 1

This should do the trick:

select a.columnA
from tablename a
inner join (select columnB, columnC
from tablename
group by columnB, columnC
having count(distinct columnD) > 1) b on a.columnB = b.columnB and a.columnC = b.columnC

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The inside join selects the rows (columnB and columnC) that have more that one (distinct) columnD, when grouping all rows from tablename by columnB and columnC.

The outer join will then select the ids (columnA) that have the columnB and columnC resulting from the inside join.