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

Foreign key referencing gastropod from slug and snail with a unique index on the foreign key columns enforces rules 2 and 3. Rule 1 is trickier though :-(

The only way I know of to enforce rule 1 is to write some database code that checks snail and slug for the presence of a row.

By the way - how do you intend to insert data? Whatever order you do it in, you will break a rule.