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

I'd go with

DROP TABLE GASTROPOD PURGE;
DROP TABLE SNAIL PURGE;

CREATE TABLE GASTROPOD
  (GASTROPOD_ID NUMBER,
  GASTROPOD_TYPE VARCHAR2(5),
  SNAIL_ID NUMBER,
  SLUG_ID NUMBER,
  CONSTRAINT GASTROPOD_PK PRIMARY KEY (GASTROPOD_ID),
  CONSTRAINT GASTROPOD_TYPE_CK CHECK (GASTROPOD_TYPE IN ('SLUG','SNAIL')),
  CONSTRAINT GASTROPOD_SLUG_CK CHECK 
     (SNAIL_ID IS NOT NULL OR SLUG_ID IS NOT NULL),
  CONSTRAINT GASTROPOD_SNAIL_CK1 CHECK 
     (GASTROPOD_TYPE = 'SNAIL' OR SLUG_ID IS NULL),
  CONSTRAINT GASTROPOD_SLUG_CK1 CHECK 
     (GASTROPOD_TYPE = 'SLUG' OR SNAIL_ID IS NULL),
  CONSTRAINT GASTROPOD_SNAIL_CK2 CHECK (SNAIL_ID = GASTROPOD_ID),
  CONSTRAINT GASTROPOD_SLUG_CK2 CHECK (SLUG_ID = GASTROPOD_ID),
  CONSTRAINT GASTROPOD_SNAIL_UK UNIQUE (SNAIL_ID),
  CONSTRAINT GASTROPOD_SLUG_UK UNIQUE (SLUG_ID)
  );

So you check that a gastropod is a snail or slug and either slug_id or snail_id is set. If it is a snail, then slug_id must be null, and for a slug then snail_id must be null. Make sure slug and snail ids are unique (I've added checks to match them to gastropod_id too).

CREATE TABLE SNAIL
  (SNAIL_ID NUMBER,
   CONSTRAINT SNAIL_PK PRIMARY KEY (SNAIL_ID),
   CONSTRAINT SNAIL_FK FOREIGN KEY (SNAIL_ID)
     REFERENCES GASTROPOD (SNAIL_ID));

Snails must point to a row in gastropod where snail_id is not null, and it is also the primary key (and therefore unique)

ALTER TABLE GASTROPOD ADD CONSTRAINT SNAIL_GS_FK FOREIGN KEY (SNAIL_ID)
     REFERENCES SNAIL (SNAIL_ID) DEFERRABLE INITIALLY DEFERRED;

Gastropods with a snail_id set must also have a corresponding row in snail. I've made this direction deferrable, otherwise you'll never get any new data it.