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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
Hi,

>>>>>>>>>>>> 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

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

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