NULL is always not equal NULL in SQL, and if this UNIQUE index isn't NOT
NULL, then this behavior is right.
-----Mensaje original-----
De: Dan Nelson [mailto:dnelson@stripped]
Enviado el: viernes, 01 de febrero de 2002 6:00
Para: Carsten Gehling
CC: Jude Insley; mysql@stripped; Paul DuBois
Asunto: Re: Bug in UNIQUE?
In the last episode (Feb 01), Carsten Gehling said:
> > >The SQL below illustrates what I believe is a bug in MySQL up to
> > >and including 3.23.47. Essentially I need a unique key where one or
> > >more of the component fields of the unique key can be NULL. What
> > >seems to happen is that you can add "duplicate" rows if the value
> > >is NULL.
> >
> > This is how UNIQUE indexes work in MySQL. All values except NULL
> > must be unique. If you use a PRIMARY KEY instead, this won't
> > happen. Of course, the reason it won't happen is that PRIMARY KEY
> > disallows NULL entirely.
>
> I thought that UNIQUE indexes constituted what is known in the
> relational model as a "candidate key" and, AFAIR, none of the members
> in a candidate key may contain NULL values.
a UNIQUE index must also have the NOT NULL attribute to be a candidate
key. Otherwise it's just an index.
--
Dan Nelson
dnelson@stripped
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