Constraint Syntax Question - Mailing list pgsql-sql

I have a many-to-many relationship between A & B where a B *must* relate to 
at least 1 A but an A may have zero Bs.

How do add this using an ALTER TABLE statement?

Thanks,

A.

--
Adam Sherman
Tritus CG Inc.
http://www.tritus.ca/
+1 (613) 797-6819