What is a primary key?

A primary key is a fundamental concept in database design and is used to identify each row in a table. In relational databases, the primary key acts as a unique identifier that ensures each record is distinguishable from the others. This is crucial for maintaining data integrity and ensuring that no duplicates occur in the database.

A primary key can consist of one or more columns, depending on how the data is structured. The most common form is a single column, typically an automatically generated ID or a unique serial number. In some cases, a composite key consisting of multiple columns can be used to create a unique identifier.

Properties of a primary key

For a primary key to be effective as an identifier, it must fulfil a number of criteria:

• Uniqueness: Each value in the primary key must be unique throughout the table, so no two rows have the same key.
• Non-null: A primary key should never contain NULL values, as this would make it impossible to uniquely identify a row.
• Immutable: A primary key should not be changed after it has been assigned to a record, as this can cause referential integrity issues in the database.
• Minimal: A primary key should only contain the necessary columns to ensure unique identification, without redundant attributes.

Different types of primary keys

There are several types of primary keys, depending on the design and needs of the database:

• Natural keys: Consist of existing data fields that are already unique, such as a social security number or an email address.
• Surrogate keys: An automatically generated value, often a unique ID or sequential numbering, assigned to each record.
• Composite keys: Consists of two or more columns that together create a unique identification of a record. This is often used in connection tables in many-to-many relationships.

The importance of a primary key in database design

A primary key is essential for maintaining data integrity and efficiently referencing data across multiple tables. When a table has a primary key, other tables can create relationships to it through foreign keys, enabling normalisation and structuring of data.

Without a primary key, a database would struggle to organise information correctly, which could lead to redundancy and inconsistencies in the data. A well-defined primary key ensures that the database functions optimally and can efficiently perform searches, updates and relationships between tables.

Examples of using primary keys

Imagine a database for a webshop where one table contains customer information. The table could be structured like this:

In this example, the CustomerID is a unique identifier for each customer. This makes it possible to reference the customers in other tables, for example an order table where the CustomerID is used as a foreign key to link orders to the correct customer.

Benefits of using a primary key

A well-chosen primary key contributes to several benefits in a database:

• Efficient data management: Search and update operations are faster as each record can be uniquely identified.
• Improved data integrity: Ensures that no duplicates exist in the table and that relationships between tables are accurate.
• Easy referencing: Allows other tables to reference records in a structured way through foreign keys.
• Improved normalisation: Helps structure the database so that data storage is more efficient and free from redundancy.

Considerations when choosing a primary key

When designing a database, it's important to choose the right type of primary key. Natural keys can be useful, but they can also have drawbacks if their values change over time. Therefore, many people choose to use surrogate keys, such as an automatically generated ID, to ensure stability.

It's also important to ensure that the primary key doesn't become too complex. A composite key may be necessary in some cases, but too many columns can make the database more difficult to manage.

By choosing an appropriate primary key, you can ensure that the database works efficiently and is easy to maintain.

FAQ - Frequently asked questions