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

"@Erwin I'd much prefer solutions that do not involve triggers - I have a pathological aversion to them."

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As far as I can see, you might be able to get away with "just using deferred constraints" in your particular case, owing to the nature of the constraint you wish to impose. If it works for you, and you are satisfied, then all is OK, no ?

My main point is, constraints (as in : "any imaginable business rule you might run into as a database designer") can get arbitrarily complex. Think of a genealogy database in which you want to enforce the rule that "no person can be an ancestor of himself, IN WHATEVER DEGREE" (that's my favourite example because it ultimately involves transitive closure and/or recursion). There is NO WAY you can get an SQL DBMS to enforce such rules without using triggers (or without using recursive SQL inside the trigger for that matter too, by the way).

Neither your DBMS nor I nor anyone else skilled in relational theory will care a Freudian shit about whatever pathologies you happen to have. But perhaps because of these pathologies that you mention, it might be interesting to observe that you can do all of the stuff you want, without having to define any triggers, if you use the DBMS I have developed myself (it does support trigger-like things, but you're not required to resort to them for enforcing data integrity).