How to correctly design composite keys and when is it better to use surrogate identifiers?

Choosing the right primary key strategy is a fundamental decision when designing any relational database. Developers often face a choice between natural keys, which consist of real business attributes, and surrogate identifiers, which carry no semantic meaning and are automatically generated by the system itself.

Surrogate keys are most commonly implemented as auto-incrementing integers or globally unique UUID identifiers. Their main advantage lies in versatility, simplicity of modifying business logic, and the stability of relationships between tables, since the artificial identifier never changes during the application's lifecycle.

Natural keys are built upon unique characteristics of a real-world object, such as an email address, passport serial number, or a combination of currency and country codes. Using natural keys automatically guarantees compliance with business constraints at the database level and eliminates the need to introduce redundant surrogate columns.

Composite keys, consisting of two or more columns, are frequently used in many-to-many relationship tables or for modeling hierarchical data. When designing composite keys, the column order is critically important:

In modern web development practice, surrogate identifiers have become the de facto standard for most tables because they simplify scaling and migrations. Nevertheless, for reference tables and junction tables, natural or composite keys remain an excellent tool for ensuring strict data integrity.