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  1. PO
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    <p>IMHO, this basically is a normalisation problem. The column named "id" does not uniquely address a row, so it can never be a PK. At least a new (surrogate) key(element) is needed. The constraint itself cannot be expressed as an expression "within the row", so it has to be expressed in terms of a FK.</p> <p>So it breaks down into two tables: One with PK=id, and a FK REFERENCING two.sid</p> <p>Two with PK= surrogate key, and FK id REFERENCING one.id The original payload "value" also lives here.</p> <p>The "one bit variable" disappears, because it can be expressed in terms of EXISTS. (effectively table one points to the row that holds the token)</p> <p>[I expect the Postgres rule system could be used to use the above two-tables-model to emulate the intended behaviour of the OP. But that would be an ugly hack...]</p> <p>EDIT/UPDATE:</p> <p>Postgres supports partial/conditional indices. (don't know about ms-sql)</p> <pre><code>DROP TABLE tmp.one; CREATE TABLE tmp.one ( sid INTEGER NOT NULL PRIMARY KEY -- surrogate key , id INTEGER NOT NULL , status INTEGER NOT NULL DEFAULT '0' /* ... payload */ ); INSERT INTO tmp.one(sid,id,status) VALUES (1,1,0) , (2,1,1) , (3,1,0) , (4,2,0) , (5,2,0) , (6,2,1) , (7,3,0) , (8,3,0) , (9,3,1) ; CREATE UNIQUE INDEX only_one_non_zero ON tmp.one (id) WHERE status &gt; 0 -- "partial index" ; \echo this should succeed BEGIN ; UPDATE tmp.one SET status = 0 WHERE sid=2; UPDATE tmp.one SET status = 1 WHERE sid=1; COMMIT; \echo this should fail BEGIN ; UPDATE tmp.one SET status = 1 WHERE sid=4; UPDATE tmp.one SET status = 0 WHERE sid=9; COMMIT; SELECT * FROM tmp.one ORDER BY sid; </code></pre>
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    1. POChecking sql unique value with constraint
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    1. USwildplasser
    1. USwildplasser
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    1. COThis example will not run in Sql 2005.
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      1. USJeremy Holovacs
    2. COI know, but 4 out of 5 of the OP's tags are not plaform-specific. Moreover, the underlying problem is more of a data modelling kind of problem than a "solve my problem on my platform" thing. For data modelling stuff it is IMHO always good to know that different solutions could exist on different implementations.
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      1. USwildplasser
 

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