Relational databases: primary key

The idea

A primary key is a field (or set of fields) in a table that uniquely identifies each record, so no two rows share the same key value and the key cannot be empty. In a table of library books, a BookID field could act as the primary key because each book has a distinct identifier, whereas a Title field would not, since two books could share a title. The primary key enforces entity integrity and is used to link tables: a foreign key in another table refers to the primary key to create a relationship. Choosing a suitable primary key means picking a field guaranteed to be unique and stable, such as an assigned ID rather than a name that might change or repeat. A candidate question asks you to identify which field could serve as a primary key, so check uniqueness and non-null requirements.

What earns the marks

• A primary key uniquely identifies each record in a table.
• No two records may share the same primary key value, and the key cannot be empty.
• A suitable primary key is a field guaranteed unique and stable, such as an assigned ID.
• A field such as a name or title is usually unsuitable because values can repeat or change.
• A primary key can be referenced by a foreign key in another table to form a relationship.
• The primary key helps enforce entity integrity and prevents duplicate records.

Common mistakes and how to put them right

• Choosing a field such as a name or title as a primary key.

  Put it right: Such values can repeat or change, so an assigned unique ID is preferred.

• Believing a primary key may be left blank.

  Put it right: A primary key must always hold a unique, non-null value.

• Confusing a primary key with a foreign key.

  Put it right: A primary key uniquely identifies records in its own table, while a foreign key refers to a primary key in another table.

Exam tips

• Test each candidate field for uniqueness across all records before selecting it.
• Reject fields whose values could change or repeat, such as names or dates.
• Remember the primary key must be non-null as well as unique.