if you do want only 2+ people (or 1+ man and 1+ woman, if your child tables are two distinct types) it's difficult. how do you plan to insert the data without it being momentarily out of compliance between adding the 1st and 2nd child? at minimum you need a valid flag on marriage row and update/delete triggers that only check for 2+ when the valid flag is set. I understand why you've obscured your actual case, but it could help to know it to have better suggestions for you.