Re: special integrity constraints

On Mon, Mar 22, 2004 at 10:20:31 +0100, Erik Thiele <erik@thiele-hydraulik.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).