Am I right with my supertype/subtype tables creation

Question

hi there,
i have subtype/supertype relationship with a disjoint rule and
partial specialisation
---table supertype
create table gift(
gift_id number,
constraint pk_gift primary key (gift_id));
---table subtypes
create table stock(
gift_id number not null,
no_of_shares number,
curr_price number,
constraint pk_gift primary key (gift_id)),
constraint uk_gift unique (gift_id)delete cascade);
create table property(
gift_id number not null,
prop_address1 varchar2(5),
prop_address2 varchar2(15),
prop_address3 varchar2(20),
post_code varchar2(8)'
curr_prop_value number,
constraint pk_gift primary key (gift_id)),
constraint uk_gift unique (gift_id)delete cascade);
create table cash(
gift_id number not null,
amount number,
constraint pk_gift primary key (gift_id)),
constraint uk_gift unique (gift_id)delete cascade);
create table artwork(
gift_id number not null,
description varchar2(30),
artist varchar2(10),
curr_art_value number,
constraint pk_gift primary key (gift_id)),
constraint uk_gift unique (gift_id)delete cascade);
could you point out any mistakes in terms of unique constraints
and gift_id appearing as inherited by the subclasses from the
superclass. Notice gift_id is inherited by the sub classes as
primary key and not foreign key.
Are there other ways of doing this assuming i am wrong?
Much appreciation.
Ayo

Answer 1

Your unique constraints are unnecessary as primary key
constraints enforce uniqueness.
You should have foreign key contraints to indeicate that GIFT is
the master of the sub-type tables...
...having said which: what is the point of GIFT - it has no
attributes of its own so it is completely unnecessary.
The fact that th esub types have no key of their own also makes
me nervous. I would expect all subtypes to have a composite
primary key eg. GIFT_ID, STOCK_NO.
Furthermore, can a GIFT be both (say) STOCK and PROPERTY? If
not then you will need to enforce that exclusivity, which is
quite difficult to do. It's easier to do if you implement the
subtypes in one table, thus:
create table gift(
gift_id number,
GIFT_TYPE varchar2(5),
stk_no_of_shares number,
stk_curr_price number,
prop_address1 varchar2(5),
prop_address2 varchar2(15),
prop_address3 varchar2(20),
prop_post_code varchar2(8)'
prop_curr_value number,
cash_amount number,
art_description varchar2(30),
art_artist varchar2(10),
art_curr_value number,
constraint pk_gift primary key (gift_id));
You can then create a check constraint that ensures that only
columns relating to stock are filled when GIFT_TYPE = 'STK',
artwork when GIFT_TYPE = 'ART', etc. Note the use of prefixes
to make it easier to see which columns are for which type.
This will impose a slight overhead in terms of space usage, but
not as much as you think - empty varchar2 columns don't take up
any space and you're saving the overheads of multiple tables.
You could save additional space by amalgamating the value/amount
columns into a single gift_value column.
This implementation is not definitely better - these things
often depend on what you're going to do with the tables and
which business rules you need to enforce. If a gift can consist
of stocks and cash then you should go for the multi-table
approach otherwise go for a single table approach.
regards, APC