Thread: Foreign key with check?

Given a table like this:

create table primary ( a integer primary key, b boolean
);

And another like this:

create table secondary ( a integer, some_other_fields
);


I would like a foreign key constraint on the "secondary" table that
looks something like:

foreign key (a, true) references primary (a, b)

That is, a row in "secondary" is allowed to reference a row in "primary"
if and only if that referenced row has (b = true).

I think I have to do this with an additional trigger. Any other options?

On Wed, Jul 27, 2005 at 16:08:19 -0400, Jeff Boes <jeff@endpoint.com> wrote:
> Given a table like this:
> 
> create table primary (
>   a integer primary key,
>   b boolean
> );
> 
> And another like this:
> 
> create table secondary (
>   a integer,
>   some_other_fields
> );
> 
> 
> I would like a foreign key constraint on the "secondary" table that
> looks something like:
> 
> foreign key (a, true) references primary (a, b)
> 
> That is, a row in "secondary" is allowed to reference a row in "primary"
> if and only if that referenced row has (b = true).

If what you are saying is that all entries in secondary need to reference
a row in primary and in addition the referenced row must have b true, then
you can do this fairly simply using a bit of extra space.
Add a row b to secondary with a constraint that b = true. Then make the
foreign key reference on both a and b.