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From "Bernt M. Johnsen" <Bernt.John...@Sun.COM>
Subject Re: unique columns must be nullable ?
Date Tue, 12 Dec 2006 10:33:28 GMT

>>>>>>>>>>>> Michael Bouschen wrote (2006-12-12 10:36:21):
> Hi,
> I ran into the same problem (not in the context Hibernate): I tried to 
> define a unique index on a nullable column which does not work with 
> derby. Here is what the derby doc says in the CONSTRAINTS section: 
> UNIQUE  Specifies that values in the column must be unique. NULL values 
> are not allowed. (see 
> http://db.apache.org/derby/docs/dev/ref/rrefsqlj13590.html.
> Is there any specific reason for not supporting unique for a nullable 
> column? I find it useful to be able to specify a column may or may not 
> have a value, but if the value is set it must be unique.

This is an open source project, so I guess the correct answer is "no
one has done it, yet". Seriously: The current index mechanism is not
able to handle null values, so to implement it is not a trivial
undertaking. Anyway, I first step would be to craete a Jira issue for
this (I believe that has not yet been done). That would at least make
the wish for the feature more visible.


Bernt Marius Johnsen, Database Technology Group, 
Staff Engineer, Technical Lead Derby/Java DB
Sun Microsystems, Trondheim, Norway

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