Re: diff. between Primary-Key and unique index

Hi

Well, Jan-Erik is right. The difference between primary key and unique key
is that unique key can be null (and NOT only once, but you can insert rows with as many null values as you want ). You can also change a unique index to "Primary key" (without PK-constraint) defining the column as NOT NULL.

rgrds Kari Pannila

      DBA - OCP Tapan Trivedi wrote:
>
> WOW!
>
> Jan-Erik Rosinowski wrote:
>
> > >if a column is declared as a unique key it will allow null
> > >value in that column (but only once) since the column is unique and
> > >therefore null values can occur but only once
> >
> > from my point of understanding 'null' is similar to infinity in
> > mathematics. nothing can be equal to infty. for that very reason null
> > IS null but (null=null) IS null..
> >
> > so i don't understand how null can be really unique or why on the
> > other hand, null isn't allowed in a pk (w/ the same semantics)
> >
> > anybody any further insights?
> >
> > ciao, jan
> >
> > http://www.rsp.de - Software zur effizienten Erstellung und Verwaltung von
> > Gutachten, Analysen, Pruefprotokollen etc.
Received on Mon Jan 24 2000 - 14:22:19 CST