Re: special integrity constraints

On Mon, Mar 22, 2004 at 10:20:31 +0100,
Erik Thiele <erik(at)thiele-hydraulik(dot)de> wrote:
> hi,
>
> i have two tables
>
> create table a (x integer);
> create table b (y real);
>
> i have the special constraint that the sum of the number of rows in
> table a plus the number of rows in table b must be even.

Does the above mean that the number of rows in a is equal to the number
of rows in b or that their sum is divisible by 2?

> so there is a posibility of adding one element to a and one element to
> b, and again the constraint is met.
>
> but this of course does not work, since between adding the element to a
> and adding the element to b, the constraint is not met.
>
> so i cannot use a trigger.
>
> what i need is the execution of a check procedure at commit time. is
> that somehow possible?

Do the real tables have candidate keys? If so you can use foreign key
references to pair a row in a to a row b (though I am not sure this
is what your real constraint is).