Still one questions though:
I tried this over multiple worksheets, where the table is on worksheet1 and
the formula in worksheet 2:
=SUMPRODUCT(--(WS1!A1:A14=1);--(WS1!B:B="b");--(WS!C:C<9);WS1!D1:D14)

The following statements give errors:
--(WS1!B:B="b")
--(WS!C:C<9)

What am I doing wrong here?