From: "Paul DuBois" <paul@stripped>
To: "Jude Insley" <jude@stripped>; <mysql@stripped>
Subject: Re: Bug in UNIQUE?
> >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.
- Carsten
