Collection of different types of data in a relational database

Question

Let's say I have a table Cars (with columns such as model, make, year, etc.), a table Books (with columns such as name, author, ISBN, etc.) and a table Wines (with columns such as name, appellation, region, vintage, etc.)

Now, even if these tables don't seem related at all, I want to make a collection of all these things called "My favorites things in the world". In this list, I will find cars, books and wines.

If I only wanted to have lists of cars, I would create a table called Car_Lists with id and name + a table called Car_List_Items with id, car_list_id (foreign key to the id in the table Car_Lists) and car_id (foreign key to the id in the table Cars).

Here, as I want to have a list of different types of things, I don't really see how this could be done.

So my question: How would I represent that with a relational database?

Answer 1

There are many ways to do it, depending on the requirements. That being said, this sounds like a homework question which is designed to make you think about it.

However, one way to do it would be to create a table that contains columns that occur in all the different types. Then create a table for each unique type that contains information specific to that type.

Then in your list table you can just create a foreign key to the base table (in this example "things")

create table "things"
(
    id int identity not null primary key
    -- add shared columns here
);
create table "cars"
(
    id int not null,
    -- add car details here
    constraint fk_cars_id foreign key (id) references "things" (id)
);
create table "myfavoritethings"
(
     id int not null,
     constraint fk_myfavoritethings foreign key "things" (id)
);

I prefer this approach because you can maintain referential integrity. Also, you are able to create a nice view from it like so knowing that the id's are unique across all type tables:

create view "vw_myfavoritethings"
as
select t.id
     , -- add car fields here
  from "things" as t
  join "cars" as c on c.id = t.id
  -- additional joins to type tables
  join "myfavoritethings" as mft on mft.id = t.id;

Another way you could do it would be to just create your individual tables and then just store the id's and type in your list table like so:

create table "myfavoritethings"
(
    id int not null primary key
);

If you use this design it would be recommended to create a sequence and then use that to generate your primary key id's across all type tables to ensure that the id value is unique. You can of course use auto increments/identities on each table but then you would need to work out what id = 1 refers to in your list yourself.