So, since you listed ‘sex’ as a unique attribute for both cats and dogs, I think you need to alter your perspective. We want to model the relationships between

So, since you listed ‘sex’ as a unique attribute for both cats and dogs, I think you need to alter your perspective. We want to model the relationships between entities, not necessarily represent OOP ‘objects’ in a DB. So, you should have an ‘animal’ table that would have fields for any attribute that is shared by all animals, such as ‘sex’, ‘medical record’, and ‘weight’ (I am assuming all animals will have a gender). Medical record would be a lookup key, probably not an integer, so you cannot enumerate or get an off by one error, use a UUID or hashid. I am assuming non-pets can have medical records. Your dog and cat tables would reference the animal table via a foreign key for those details. For cat type and dog type, I would make those foreign key references to lookup tables for each of the respective types. Animal table would also have a foreign key reference to the pet table, but this can be NULL, if the animal is not a pet. Thoughts?