Re: Unique constraint and NULL values

From: David Portas <REMOVE_BEFORE_REPLYING_dportas_at_acm.org>
Date: Thu, 21 Oct 2004 23:49:08 +0100
Message-ID: <1rSdnf-sNa16oeXcRVn-tw_at_giganews.com>

> yes, in theory, the SQL Server approach is more 'pure'

Not really. The SQL standard consistently defines all the nullable constraints (CHECK, UNIQUE, FOREIGN KEY) in the negative. In other words, the constraint doesn't permit violations (logical FALSE) but nor does it require that the constraint be satisfied (logical TRUE). The UNKNOWN case is not considered a violation of a constraint, which is why NULLs are permitted. Unfortunately Microsoft disagrees ;-)

-- 
David Portas
SQL Server MVP
--

Received on Fri Oct 22 2004 - 00:49:08 CEST

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