On Wed, 2005-04-27 at 09:06, Stephane Bortzmeyer wrote:
> On Wed, Apr 27, 2005 at 05:19:32AM +0000,
> Patrick TJ McPhee <ptjm@interlog.com> wrote
> a message of 37 lines which said:
>
> > but you should know that in SQL, unique constraints don't apply to
> > rows containing null values
>
> May be I should but I didn't.
Actually, considering that many databases (at least in the past) have
ignored this and treated nulls as unique things, it's quite
understandable.
MSSQL, for instance, used to definitely allow only one null in a unique
field. So, for that database, not null wasn't really necessary for a
primary key column.
I believe this problem exist(s)(ed) in several other supposedly
"enterprise" class databases as well.