What is a primary key and foreign key in databases?

Primary Key — is a unique identifier for a record in a database table. It guarantees that each row in the table can be uniquely identified. Usually, a primary key cannot contain NULL and must be unique.

Foreign Key — is a field (or set of fields) in one table that references the primary key of another table. A foreign key is used to establish and enforce referential integrity between two tables, meaning it guarantees that the value in this field exists in the related table.

Example:

• The orders table has a customer_id field, which is a foreign key referencing the id field in the customers table (the primary key).

This allows linking an order to a specific customer and ensures data integrity — you cannot add an order with a non-existent customer.