A foreign key value can be null, if it suits the data application - or that
is what I've always been taught.

Here is a text book definition of Referential Integrity which was spoon fed
to me by the Open University when studying their RDBMS course:

"Referential Integrity - If a relation (table), R2 has a foreign key, F,
that references the primary key, P, in another relation (table), R1, then
every R2.F entry must either be a value equal to an R1.P primary key value
or be null"

Regards,
Chris

> -----Original Message-----
> From: Bob Hall [mailto:[EMAIL PROTECTED]]
> Sent: 31 January 2001 01:53
> To: [EMAIL PROTECTED]
> Subject: Re: [PHP-DB] foreign key problem
>
>
> >hello all!
> >i have a little problem ,hope someone can help me out
> >the problem is :
> >let i have two tables T1 and T2 ,now T1 has following
> >fields
> >cntryid
> >cntryname
> >cntrycode etc.
> >now cntryid is the primary key now i want to make the
> >cntryid a foreign key in table T2 .so how can i do it
> >plz help me .
> >msjamal
>
> A column is a foreign key because it contains only values found in
> the referenced key, and no NULLs. Design your database so that the
> foreign key column contains only values from cntryid.
>
> If you want to know how to declare a referenced key/foreign key
> relationship, you will have to specify what RDBMS you are using.
>
> Bob Hall
>
> Know thyself? Absurd direction!
> Bubbles bear no introspection.     -Khushhal Khan Khatak
>
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