CREATE TABLE categories (
      category_id integer not null primary key,
      category char varying(20) not null unique);

CREATE TABLE products (
      product_id integer not null primary key,
      product varchar(20) not null unique);

CREATE TABLE items (
      item_id integer not null primary key,
      product_id integer not null references products(product_id));

CREATE TABLE book_titles (
      item_id integer not null primary key references items(item_id),
      authors_list varchar(20) not null,
      isbn char(10) not null unique);

INSERT INTO categories (category_id, category) VALUES (1, 'Music');
INSERT INTO categories (category_id, category) VALUES (2, 'Books');
INSERT INTO categories (category_id, category) VALUES (3, 'Video');

INSERT INTO products (product_id, product) VALUES (1, 'Arn');
INSERT INTO products (product_id, product) VALUES (2, 'Databasteknik');
INSERT INTO products (product_id, product) VALUES (3, 'Compilers');

INSERT INTO items (item_id, product_id) VALUES (10, 1);
INSERT INTO items (item_id, product_id) VALUES (20, 2);
INSERT INTO items (item_id, product_id) VALUES (30, 3);

INSERT INTO book_titles (item_id, authors_list, isbn)
values (10, 'Guillou', '1111111111');
INSERT INTO book_titles (item_id, authors_list, isbn)
values (20, 'Padron-McCarthy, Ri', '2222222222');
INSERT INTO book_titles (item_id, authors_list, isbn)
values (30, 'Elmasri, Navathe', '3333333333');

select isbn, authors_list as authos, product as title
from book_titles join items using(item_id) join products using(product_id);