SQL | Constraints

Last Updated : 8 Sep, 2026

SQL Constraints are rules applied to columns in a table to restrict the type of data that can be stored. They help maintain data accuracy, consistency and integrity in a database.The commonly used SQL Constraints are:

  • NOT NULL
  • UNIQUE
  • PRIMARY KEY
  • FOREIGN KEY
  • CHECK
  • DEFAULT

Types of SQL Constraints

The following are the commonly used SQL constraints:

NOT NULL Constraint

The NOT NULL constraint ensures that a column cannot contain NULL values. It is used when a value is required for every row.

Syntax:

CREATE TABLE employee (
    employee_id INT NOT NULL,
    name VARCHAR(50) NOT NULL
);

Here, employee_id and name cannot contain NULL values.

Screenshot-2026-09-07-105134

UNIQUE Constraint

The UNIQUE constraint ensures that all values in a column are different. It prevents duplicate values from being stored.

Syntax:

CREATE TABLE employee (
    employee_id INT,
    email VARCHAR(100) UNIQUE
);
Screenshot-2026-09-07-112558
  • Here, each employee must have a unique email address.

PRIMARY KEY Constraint

The PRIMARY KEY constraint uniquely identifies each row in a table. A primary key cannot contain NULL or duplicate values.

Syntax:

CREATE TABLE employee (
    employee_id INT PRIMARY KEY,
    name VARCHAR(50)
);
Screenshot-2026-09-07-111144
  • Here, employee_id uniquely identifies each employee.

FOREIGN KEY Constraint

The FOREIGN KEY constraint is used to establish a relationship between two tables. It ensures that the value in one table refers to a valid value in another table.

Syntax:

CREATE TABLE employee (
    employee_id INT PRIMARY KEY,
    department_id INT,
    FOREIGN KEY (department_id)
        REFERENCES department(department_id)
);
Screenshot-2026-09-07-111144
  • Here, department_id in the employee table references department_id in the department table.

CHECK Constraint

The CHECK constraint ensures that the values in a column satisfy a specified condition.

Syntax:

CREATE TABLE employee (
    employee_id INT,
    age INT CHECK (age >= 18)
);
Screenshot-2026-09-07-111504
  • Here, the age must be greater than or equal to 18.

DEFAULT Constraint

The DEFAULT constraint automatically assigns a specified value to a column when no value is provided.

Syntax:

CREATE TABLE employee (
    employee_id INT,
    department VARCHAR(50) DEFAULT 'HR'
);
Screenshot-2026-09-07-111640
  • If no value is provided for department, HR is assigned automatically.

Example

The following example demonstrates how multiple constraints can be applied to a table:

CREATE TABLE employee (
    employee_id INT PRIMARY KEY,
    name VARCHAR(50) NOT NULL,
    email VARCHAR(100) UNIQUE,
    age INT CHECK (age >= 18),
    department VARCHAR(50) DEFAULT 'HR'
);

Output:

Screenshot-2026-09-07-105453
  • PRIMARY KEY uniquely identifies each employee.
  • NOT NULL ensures that name must have a value.
  • UNIQUE prevents duplicate email addresses.
  • CHECK ensures that age is at least 18.
  • DEFAULT assigns HR when no department is specified.
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