Topic 9: Databases — Part 3

03 Primary keys

A primary key is a field (or combination of fields) that provides a unique identifier for each record in a table. No two records in the same table can share the same primary key value — every record must be distinguishable from every other.

The purpose of a primary key is to:

• Uniquely identify each record so it can be found, updated, or deleted accurately
• Link tables in a relational database — another table stores the primary key value as a foreign key to create a relationship
• Protect data integrity by preventing duplicate records

A field can only be chosen as a primary key if it meets these selection criteria:

• The value must be unique for every record
• The value must never change for a given record (it should permanently identify that record)
• The value must always be present — it cannot be blank or NULL

Consider a library Books table. Which fields are suitable as a primary key?

If no existing field in a table is unique, you must add a new field specifically to serve as the primary key — for example, an auto-incrementing MemberID in a library members table where names and addresses could be duplicated.