SQL NOT NULL constraint

Here is a clear, practical, and complete guide to the SQL NOT NULL constraint, including behavior, examples, and engine differences.

What the SQL NOT NULL constraint Does

NOT NULL ensures that a column cannot contain NULL values.

• Prevents missing or unknown data

• Enforces data integrity at the database level

• Often used for required fields (email, username, timestamps, etc.)

Basic Syntax

In CREATE TABLE:

In ALTER TABLE:

 Example: Ensure a required column

•  Prevents inserting a product without name or price.

 Inserting Rows With NOT NULL Columns

Valid:

Invalid (missing required field):

 Add NOT NULL to an Existing Column (with data present)

PostgreSQL:

MySQL:

SQL Server:

•  Must remove NULLs first or the ALTER will fail.

Remove NOT NULL Constraint

PostgreSQL:

MySQL:

SQL Server:

Important Behavior Notes

• NOT NULL prevents NULL but not empty strings (''), unless you enforce that separately.
•  Most engines treat PRIMARY KEY as implicitly NOT NULL.
•  Columns without an explicit constraint default to NULL unless changed.
•  For mandatory columns, combine with CHECK or DEFAULT when helpful.

Best Practices

•  Apply NOT NULL to all required fields
•  Add sensible defaults (e.g., DEFAULT CURRENT_TIMESTAMP)
•  Clean existing NULL data before adding constraints
•  Avoid allowing NULL on foreign key columns unless logically required
•  Use migrations to apply changes safely in production