What is the best way to enforce a 'subset' relationship with integrity constraints

Question

For example, given 3 tables:

• gastropod
• snail
• slug

and assuming we want to enforce that

1. every row in \'gastropod\

Answer 1

"and assuming we want to enforce that (1) every row in 'gastropod' has exactly one corresponding row in 'snail' or 'slug' (but not both) (2) every row in 'slug' has exactly one corresponding row in 'gastropod' (3) every row in 'snail' has exactly one corresponding row in 'gastropod'"

(1) is an inclusion dependency (aka "foreign key dependency") between 'GASTROPOD' and a virtual relvar (aka "view") defined as SLUG UNION SNAIL. (2) and (3) are the same kind of inclusion dependencies between 'SLUG' (/'SNAIL') and 'GASTROPOD'.

All of them taken together mean that you have an "equality dependence" between 'GASTROPOD' and 'SLUG UNION SNAIL' (at least as far as the identifiers are concerned).

Note that to be able update a database that is under such constraints, you are likely to need either a DBMS engine that supports this thing called "Multiple Assignment", or else one that supports "Deferred Constraint Checking".

Chapter 11 of the book "Applied Mathematics for Database Professionals" goes into great depth on the subject of how to enforce such constraints (and in fact, just any constraint, however complex) in SQL environments. The answer to your question is almost the entire contents of that chapter, and I hope you don't expect me to summarize it all here in a few words (the essence of the answer is "triggers" - as StarShip3000 also indicated).