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

My own solution for postgres (but I have no idea if it is the best way):

enum:

create type gastropod_type as enum ('slug', 'snail');

tables and constraints:

create table gastropod(
  gastropod_id serial unique,
  gastropod_type gastropod_type,
  slug_gastropod_id integer,
  snail_gastropod_id integer,
  average_length numeric,
  primary key(gastropod_id, gastropod_type),
  check( (case when slug_gastropod_id is null then 0 else 1 end)+
         (case when snail_gastropod_id is null then 0 else 1 end)=1) );

create table slug(
  gastropod_id integer unique,
  gastropod_type gastropod_type check (gastropod_type='slug'),
  is_mantle_visible boolean,
  primary key(gastropod_id, gastropod_type),
  foreign key(gastropod_id, gastropod_type) 
    references gastropod deferrable initially deferred );

create table snail(
  gastropod_id integer unique,
  gastropod_type gastropod_type check (gastropod_type='snail'),
  average_shell_volume numeric,
  primary key(gastropod_id, gastropod_type),
  foreign key(gastropod_id, gastropod_type)
    references gastropod deferrable initially deferred );

alter table gastropod 
add foreign key(slug_gastropod_id, gastropod_type) 
references slug deferrable initially deferred;

alter table gastropod 
add foreign key(snail_gastropod_id, gastropod_type) 
references snail deferrable initially deferred;

test:

insert into gastropod(gastropod_type, slug_gastropod_id, average_length)
values ('slug', currval('gastropod_gastropod_id_seq'), 100);

insert into slug(gastropod_id, gastropod_type, is_mantle_visible)
values (currval('gastropod_gastropod_id_seq'), 'slug', true);

select gastropod_id, gastropod_type, average_length, is_mantle_visible
from gastropod left outer join slug using(gastropod_id, gastropod_type) 
               left outer join snail using(gastropod_id, gastropod_type);

 gastropod_id | gastropod_type | average_length | is_mantle_visible
--------------+----------------+----------------+-------------------
            1 | slug           |            100 | t                 
(1 row)