The Hermit Hermit's Free Library  Database Design Cases

Magazine Subscriptions Database

This is the sixth in a series of database design case studies. The database in this example is constructed of four tables. It is recommended to begin with the first case study, which illustrates a two-table database.

System Analysis

A magazine distributor has approached Databased Advisors for a proposal to create a database for its subscription service.

The distributor needs two reports:

Fields

The first step in database design is to determine all the necessary fields and segregate them into groups that mirror the natural order of the real world. Here we have organized the fields into four groups. (Note that these field lists are representative and not exhaustive).

Field names for relational database tables

Tables

When the fields have been grouped into their natural order, each group represents a table. The Orders table represents the header information on an invoice or bill.

Diagram of tables in a relational database

Key Fields

Each record in a table must be uniquely identified by its value in the table's key field.

Diagram of relational database tables showing key fields

Relationships

The next step is to identify the relationships between the tables.

We determine that there is a one-to-many relationship between Publishers and Magazines: a publisher can publish many magazines, but every magazine is published by only one customer. Diagram of relationships between tables of a relational database

We can also see that there is a many-to-many relationship between Subscribers and Magazines. A subscriber can subscribe to many magazines, and each magazine can be subscribed to by many subscribers.

Primary & Foreign Keys

When two tables have a many-to-many relationship we create a third table, often referred to as a lien table because it completes the many-to-many relationship.

In this example, the Subscriptions table serves as a lien between the Subscribers and Magazine tables, so we place the Primary Keysfrom the Subscribers and Magazine tables into the Subscriptions table as foreign keys.

Likewise, we place the Primary key from the Publishers table into the Magazine table as a foreign Key to create their One-to-Many relationship.

Diagram of relationships between tables of a relational database showing primary and foreign keys

The Rationalized Model

The final result is a relational database with a many-to-many relationship between Inventory and Orders, and a ne-to-many relationship between Customers and Orders.

When a database model satisfies all the technical requirements it is said to be rationalized.

The objective of rationalizing a database structure is to eliminate redundancy in order to save disk space and to simplify updates by insuring that every piece of information exists in only one place. The only fields that repeat are the foreign keys.

A rational database model must satisfy these four rules and pass these three tests.

Complete diagram of a relational database