PostgreSQL - NOT NULL Constraint

Last Updated : 4 Aug, 2026

The NOT NULL constraint in PostgreSQL ensures that a column always contains a value and cannot store NULL.

  • Enforce mandatory fields in a table.
  • Improve data consistency and integrity.

Syntax

Create a NOT NULL constraint using the CREATE TABLE statement is:

CREATE TABLE table_name (
    column_name data_type NOT NULL
);

Add a NOT NULL constraint:

ALTER TABLE table_name
ALTER COLUMN column_name
SET NOT NULL;

Working

Example 1: Create a Table with a NOT NULL Constraint

Query:

CREATE TABLE Employees (
    EmployeeID INT PRIMARY KEY,
    EmployeeName VARCHAR(50) NOT NULL,
    Department VARCHAR(50),
    Salary NUMERIC
);

INSERT INTO Employees VALUES
(101, 'James Carter', 'Sales', 55000),
(102, 'Emily Johnson', 'Finance', 62000);

Output:

13

Example 2: Insert a NULL Value into a NOT NULL Column

Query:

INSERT INTO Employees
VALUES
(103, NULL, 'Marketing', 48000);

Output:

14

Example 3: Remove the NOT NULL Constraint from an Existing Column

Firstly, the EmployeeName column is defined with the NOT NULL constraint.

Query:

ALTER TABLE Employees
ALTER COLUMN EmployeeName
DROP NOT NULL;

Output:

15

Example 4: Insert a NULL Value After Removing the NOT NULL Constraint

Query:

INSERT INTO Employees
VALUES
(103, NULL, 'Marketing', 48000);

Output:

16
  • The INSERT statement succeeds because the NOT NULL constraint has been removed from the EmployeeName column.
  • PostgreSQL allows NULL values in the EmployeeName column after the constraint is dropped.
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