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SQL NOT NULL Constraint

The NOT NULL constraint ensures that a column cannot contain NULL values. It guarantees that a field always has a value, preventing missing or undefined data.

 

Key Features of SQL NOT NULL Constraint

  • Ensures that a column must always have a value.
  • Used in table creation or modification.
  • Prevents inserting NULL into the column.

 

SQL NOT NULL Constraint Syntax

Defining NOT NULL in Table Creation

CREATE TABLE Employees (
    EmployeeID INT PRIMARY KEY,
    Name VARCHAR(50) NOT NULL,
    Age INT NOT NULL
);
  • Name and Age must have values; NULL is not allowed.

Adding NOT NULL to an Existing Column

ALTER TABLE Employees  
MODIFY Age INT NOT NULL;
  • Makes the Age column mandatory.

 

Inserting Data into a Table with NOT NULL Constraint

INSERT INTO Employees (EmployeeID, Name, Age)  
VALUES (1, 'John Doe', 30);  -- ✅ Successful

INSERT INTO Employees (EmployeeID, Name, Age)  
VALUES (2, NULL, 25);  -- ❌ Error: Name cannot be NULL

 

Removing NOT NULL Constraint

ALTER TABLE Employees  
MODIFY Age INT NULL;
  • Allows NULL values in Age.

 

Summary

  • The NOT NULL constraint prevents missing data.
  • It can be defined at creation or added later.
  • NULL values cause errors when inserted.
  • It can be removed using ALTER TABLE.
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