What are primary key constraints in relational databases?

Primary key constraints are rules in relational databases that ensure each row in a table is uniquely identifiable. A primary key is a column (or combination of columns) that enforces two main properties: uniqueness and non-nullability. This means no two rows can have the same value in the primary key column(s), and the column(s) cannot contain empty or undefined values (NULL). For example, in a users table, a column like user_id might serve as the primary key, guaranteeing that every user has a distinct identifier. The database automatically enforces these constraints by rejecting inserts or updates that violate them, such as duplicate values or missing entries in the primary key column.

Primary keys can also be composed of multiple columns, known as a composite key. For instance, in an order_items table, combining order_id and product_id could form a composite primary key to ensure the same product isn’t added twice to the same order. While individual columns in a composite key might repeat (e.g., the same product_id appearing in different orders), their combination must remain unique. Additionally, most databases automatically create an index on the primary key column(s) to speed up lookups, making queries that filter or join on the primary key more efficient.

When designing tables, choosing an appropriate primary key is critical. Common strategies include using surrogate keys (artificial identifiers like auto-incrementing integers) or natural keys (existing data like email addresses). Surrogate keys are often preferred because they’re stable and avoid dependencies on real-world data that might change (e.g., an email address). However, natural keys can simplify certain queries if the data is inherently unique and immutable. Developers should also ensure primary keys are minimal (no unnecessary columns) and align with the table’s usage patterns. For example, using a composite key in a junction table for a many-to-many relationship ensures data integrity while reflecting the relationship’s structure.