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Date: Tue, 6 May 2003 10:26:04 -0700 (PDT)
From: chester c young <chestercyoung@yahoo.com>
Subject: Re: UNIQUE boolean: only one row may be "true"
To: Randall Lucas <rlucas@tercent.net>
Cc: pgsql-sql@postgresql.org
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> How shall I create a constraint such that only one row may be 'true'?

Instead of, or along with your pg boolean, use a C type boolean where
true is 1 and false is null:
  ...
  col_tf int check( col_tf = 1 || col_tf is null ) unique,
  ...
a kludge to be sure, but works

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