A primary key is a unique identifier for each record in a database table, ensuring that no two rows have the same value in the specified column(s) and preventing duplicate entries.

A primary key ensures that each row in a table is uniquely identifiable. It must satisfy two conditions:

1. Uniqueness – No two records can have the same primary key value.
2. Not NULL – A primary key cannot be NULL because it is used to identify records.

Primary keys can be single-column keys (one field uniquely identifies a row) or composite keys (a combination of multiple columns uniquely identifies a row).

For example, in an Employees table, the employee_id field can be the primary key because each employee must have a unique ID.

Example SQL Query (Defining a Primary Key):

CREATE TABLE Employees (
employee_id INT PRIMARY KEY,
name VARCHAR(100),
department VARCHAR(50),
salary DECIMAL(10,2)
);

This ensures that employee_id is unique for every row in the Employees table.

If a composite primary key is required, it can be defined as follows:

CREATE TABLE Enrollments (
student_id INT,
course_id INT,
PRIMARY KEY (student_id, course_id)
);

Here, the combination of student_id and course_id uniquely identifies each row in the Enrollments table.