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

One of the problems with SQL is its poor level of support for integrity constraints, especially referential constraints.

For all practical purposes your problem cannot be solved using SQL constraints unless you disable the constraints when you want to insert a row to a table. The reason is that SQL requires tables to be updated one at a time and so the constraint must be violated whenever new rows are inserted. This is a fundamental limitation of SQL and all the major DBMSs suffer from it.

There are some workarounds but none of them is perfect. You could use DEFERRABLE constraints if your DBMS has them (Oracle for example). A DEFERRABLE constraint is really just an easy way of disabling a constraint. Or you could use triggers, which means the rule is enforced procedurally rather than through a proper database constraint.