63.5 Introducing Primary Keys

Each database table should contain one or more columns that can be used to identify each row in the table uniquely. This is known in database terminology as the Primary Key. For example, a table may use a bank account number column as the primary key. Alternatively, a customer table may use the customer’s social security number as the primary key.

Primary keys allow the database management system to identify a specific row in a table uniquely. Without a primary key it would not be possible to retrieve or delete a specific row in a table because there can be no certainty that the correct row has been selected. For example, suppose a table existed where the customer’s last name had been defined as the primary key. Imagine then the problem that might arise if more than one customer named “Smith” were recorded in the database. Without some guaranteed way to identify a specific row uniquely, it would be impossible to ensure...