Re: FK issue

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On Thursday 10 June 2004 09:27 am, mike wrote:
> On Thu, 2004-06-10 at 17:03, Uwe C. Schroeder wrote:
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> > On Thursday 10 June 2004 08:21 am, mike wrote:
> > > I have set up a FK as follows
> > >
> > > ALTER TABLE lk_sub_con ADD FOREIGN KEY (type) REFERENCES
> > > lk_sort_of_contact(type_code);
> > >
> > > However when I do this
> > >
> > > INSERT INTO lk_sort_of_contact (type_code) VALUES ('1') (ie: a NULL
> > > into the FK) it works
>
> this should have said
>
> INSERT INTO lk_sub_con (sub_cat) VALUES ('1') (ie: a NULL into
> the FK) it works ie: a NULL in type
>
> (wrong copy)
>
> > > Is this a bug?
> >
> > No.
> > you have a table lk_sub_con. That table has the foreign key assigned.
> > This means it doesn't matter what you put into lk_sort_of_contact , it
> > matters what you put into lk_sub_con.
> > The foreign key says something like
> > "If you put a value into field type of table lk_sub_con, the same value
> > must be existant in table lk_sort_of_contact field type_code"
> >
> > Try to add a null value to lk_sub_con.type - or any value that's not in
> > lk_sort_of_contact. Postgres will throw an error.
>
> this is the problem - it doesnt if I put a null in (the refernced column
> has no nulls)

NULL is simply no value. A foreign key only checks for values.
Modify lk_sub_con to have a "NOT NULL" in the definition of type.
NOT NULL forces the field type to have a value and whenever there's a value
the foreign key will kick in.


    UC

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