Intersection table

Question

Hello,
I have the table customer:
CREATE table customer
cust_id NUMBER(5),
phone_number NUMBER(10) NOT NULL,
first_name VARCHAR(12) NOT NULL,
last_name VARCHAR(30) NOT NULL,
credit NUMBER(5, 2) DEFAULT 0 NOT NULL,
CONSTRAINT cust_pk PRIMARY KEY (cust_id),
CONSTRAINT phone_unique UNIQUE (phone_number)
and the table movie:
CREATE TABLE movie
title VARCHAR(30) PRIMARY KEY,
production_year NUMBER(4),
rating VARCHAR(5) NOT NULL,
max_days NUMBER(1) DEFAULT 1,
quantity NUMBER(2),
CONSTRAINT check_ratings CHECK (rating IN ('G', 'PG', 'PG-13', 'R', 'NC-17')),
CONSTRAINT check_maxDays CHECK (max_days IN (1, 3, 7))
now I want to store the rent of a movie, I come up with the following table:
CREATE TABLE cust_rent_movie
title VARCHAR(30),
cust_id NUMBER(5),
rent_date DATE DEFAULT current_date NOT NULL,
return_date DATE,
CONSTRAINT cust_rent_movie_pk PRIMARY KEY (title, cust_id, rent_date)
Is the table cust_rent_movie an intersection table. Have I to declare the title and cust_id are foreign keys, the primary key consist of an extra attribute the rent_date.
When a customer rent a movie I want to subtract one from the movie.quantity and when the customer return the movie I want to add one to the movie.quantity subtract from customer.credit
Yiannis P.

Answer 1

IPapas wrote:
Is the table cust_rent_movie an intersection table. Yes. It allows for a many to many relationship between customer and movie, though it would seem that you would want to connect customers to a table with 1 row per physical DVD. That table could then be joined to the title movie table in order to get the title.
Have I to declare the title and cust_id are foreign keys, the primary key consist of an extra attribute the rent_date.Title is not a good column to use for the primary key, consider adding title_id. Though unlikely, it is possible that the title wil be updated, ie to correct a typo. Currently you will not be able to update it as there mat be dependent transactions. You will want to add a PK for this table, ie cust_movie_pk.
When a customer rent a movie I want to subtract one from the movie.quantity and when the customer return the movie I want to add one to the movie.quantity subtract from customer.creditIt is usually better to derive values like quantity at run time (or query time) rather than trying to keep a running total of all changes. With appropriate indexes you should be able to quickly calculate that column when required.
CONSTRAINT phone_unique UNIQUE (phone_number)This will cause a problem for you if you have multiple customers in the same household with the same phone number.