Type mismatch problem

As a total beginner who has for decades used an application that would allow type mismatches like boolean*numeric I
triedthe following select: 

SELECT sum(cost*quantity)*(sales_type=1) AS sales_type1, sum(cost*quantity)*(sales_type=2) AS sales_type2 FROM etc etc

In the above, cost and quantity are TYPE numeric(9,2), sales_type is smallint.

PostgreSQL does not allow numeric*boolean.  The error message ends with "You might need t"  which kinda leaves me
hanging.

Any clues would be most welcome.


Michael Rowan
mike.rowan@internode.on.net

11 Kingscote Street
ALBERTON
South Australia 5014

tel 618 8240 3993