[GENERAL] Constraint Problem

[Ron St-Pierre]

I want to create a constraint that's a little different, but don't know 
how to implement it. I have an intermediate table with a compound PK and 
a boolean 'ysnDefault' column:

  comanyID     INTEGER REFERENCES companies,
  assocationID INTEGER REFERENCES associations,
  ysnDefault     BOOLEAN
I just want to constrain the ysnDefault field to only allow one TRUE 
value for any companyID/associationID pair, with no restrictions on the 
number of FALSES.

At first glance I thought it would be easy, but I can't see how to do 
it. Any suggestions?

Thanks
Ron


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