assertion

Hi

I have two tables, INMAT to track incoming material, which I process and
return, and OUTMAT to track outgoing material. each entry in INMAT is a
serial, INKY. Incoming Quantity is under field INQTY. Outgoing quantity
in OUTMAT is under field OQTY. To check that outgoing material does not
exceed material that came in I copied the following construction from
"Database Systems" by Elmasri and Navathe.

CREATE ASSERTION OQTY_CONSTRAINT
CHECK ( OUTMAT.OQTY<=INMAT.INQTY FROM OUTMAT,INMAT
WHERE OUTMAT.INKY=INMAT.INKY);

It did not work. I am using Postgresql-7.0.2. Can someone point out my
mistake?

Thanks in advance.

Vijay