What is a primary key and a foreign key in relational databases, and why are they important?

A primary key is a column or set of columns that uniquely identifies each row in a table. It must contain unique, non‑NULL values and is often indexed to speed up lookups. A foreign key is a column (or set) that references the primary key of another table, establishing a relationship between the two tables. The foreign key enforces referential integrity: it ensures that a value in the child table corresponds to an existing row in the parent table, preventing orphaned records. Together, primary and foreign keys provide a structured way to model real‑world relationships, enable efficient joins, and maintain data consistency across the database. They also allow the database engine to enforce constraints automatically, reducing application‑level validation logic and improving data quality.