On Wed, Feb 14, 2007 at 10:01:58AM +0100, Steven Lambert wrote:
> You are right, the index has more columns and one of them have most of the
> time the value NULL.
> It is the date when the record has been "deleted"
See "Unique Constraints" in the documentation, in particular the
"However, null values are not considered equal in this comparison.
That means even in the presence of a unique constraint it is possible
to store duplicate rows that contain a null value in at least one
of the constrained columns."
This has been discussed before; search the archives for words like
"null", "unique", and "comparison" or "compare".
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