What is a primary key?

A primary key is a column or set of columns that uniquely identifies each row in a relational database table. It must contain unique values and cannot contain NULLs. The primary key enforces entity integrity and is used as the target of foreign key relationships. It is typically implemented as a unique index, often clustered, which improves query performance. In SQL, you define a primary key when creating a table: CREATE TABLE Employees (EmployeeID INT PRIMARY KEY, Name VARCHAR(100), ...). You can also add a primary key to an existing table with ALTER TABLE. Primary keys are essential for relational modeling, ensuring data consistency, and enabling efficient joins. They also serve as the basis for many database operations such as updates, deletes, and referential integrity enforcement. In practice, choosing a good primary key is important. Surrogate keys (like auto-increment integers or UUIDs) are often used because they are stable and independent of business data. Natural keys (like SSN or email) can be used but may change over time. The primary key also influences indexing strategy; many databases automatically create a unique index on the primary key, which can be clustered or non-clustered depending on the DBMS. When designing tables, you should consider the cardinality, update frequency, and whether the key will be used in foreign key relationships. A well-chosen primary key improves data integrity, query performance, and simplifies application logic.