Is it possible that a customers asks for a product more than once?
For example: he/she asks an offering for 10,10,11,11,12?
If this is possible than solutions like
where pi in (10,10,11,11,12)
will not work.
Because 'pi in (10,10,11,11,12)' is the same as 'pi in (10,11,12)'.
A solution for 10,10,11,11,12 is A&B.