conditional FROM

Good day...

I'm trying to build a query for PGSQL 9.1 where a table has two 
references with only one being used depending of the type of entry..

For example, the table has following simplified structure:
portid        primary keyport2node    index to table nodeport2card    index to table card

So how can I do a conditional FROM clause in the query depending on the 
column port2node and port2card?
If port2card is Null or 0 I don't want it in the FROM clause as the 
query will return unneccessary duplicate
row...the same goes for port2node being Null or 0...


thanx in advance
richard