SQL Query help needed

I am new to SQL and PostgreSQL and I'm not sure how to get this done in one
query:

I have 3 tables:

Vendor
------
VendorNumber
EmailAddress
.....

CrossReference
--------------
VendorNumber
PartType
SystemType

PartRequest
--------
RequestNumber
PartType
SystemType
.....

I am processing the Parts Requests.

I need a query that returns the PartRequest.RequestNumber and
Vendor.EmailAddress so I can send the Request to all Vendors that carry the
requested PartType & SystemType combinations of parts.

So a given Request may be for a PartType of PrinterCable and SystemType of
Macintosh.

The CrossReference table has a list of all vendors that carry PrinterCables for
Macintoshes.

I am using PostgreSQL so I don't have foreign keys and I'm not sure enough of
SQL yet to know if that's a problem.

Can someone here please help me with this query?
--
Scott Chapman sends....
sec(at)proaxis(dot)com