Constraint that compares and limits field values

[MargaretGillon@xxxxxxxxxxxxxx]

I have a table that I am using to hold
keys for M:M relationships.   I have six fields that can hold the
keys and I do this because I validate the key with a foreign key constraint.
Fields evevid1, evevid2 hold keys from the event table, evreid1, evreid2
hold keys from the resource table, etc. The 0 works with the FK constraints
because in each table being referenced I have a record with id = 0 that
is empty.  Each row should only have two foreign key values and the
other key field values are 0.

How do I put a constraint on the Insert
/ Update to make sure that only two fields out of the six have a value
> 0 in them.

I want to make sure that one of these
three fields has a value > 0 and the other two are 0.
 evevid1 int4 
 evenid1 int4 
 evreid1 int4 

Same here. I want to make sure that
one of these three fields has a value > 0 and the other two are 0.
 evevid2 int4
 evenid2 int4
 evreid2 int4 
 
Please reply to me as well as the list
because I get the digest version and it might take a while to get the reply
there.

Thank you,
*** *** *** *** *** *** *** *** ***
*** *** *** *** *** *** *** *** *** *** ***
Margaret Gillon, IS Dept., Chromalloy Los Angeles, ext. 297

This e-mail message and any attachment(s) are for the sole use of the intended
recipient(s) and may contain proprietary and/or confidential information
which may be privileged or otherwise protected from disclosure.  Any
unauthorized review, use, disclosure or distribution is prohibited.  If
you are not the intended recipient(s), please contact the sender by reply
email and destroy the original message and any copies of the message as
well as any attachment(s) to the original message.