NOT NULL

The NOT NULL constraint prevents a column from storing NULL values.

In other words, the database requires a real value. Omitting the column in an INSERT or explicitly passing NULL will both trigger an error.

Why NOT NULL Matters

Many fields are critical to business logic. A missing Name, Email, or OrderDate can break downstream calculations.

NOT NULL protects these fields at the schema level.

Inserting without Name or Email fails:

Defining NOT NULL

Set it during table creation:

Add it later:

Note: Modifying an existing column to NOT NULL requires that all current rows already contain non-NULL values.

NOT NULL with DEFAULT

NOT NULL works well with DEFAULT to guarantee a value when the user supplies none.

Even if the INSERT statement omits Status, the database fills 'Active'.

Real-World Uses

Pseudocode example:

Frequently Asked Questions

Q: What is the difference between NULL and an empty string? A: NULL means unknown or missing data. An empty string '' is a known value. NOT NULL blocks NULL, but not ''.

Q: Can a PRIMARY KEY column still be NULL? A: No. A PRIMARY KEY automatically implies NOT NULL.

Q: Can I change a nullable column to NOT NULL later? A: Yes, as long as existing rows have values for that column. Otherwise the operation fails.

Summary

• NOT NULL requires a value in a column.
• It prevents incomplete records at the database level.
• Combine it with DEFAULT for safe automatic values.
• Critical fields like names, emails, and dates are common candidates.

Next Step

See UNIQUE to prevent duplicate values, or return to the SQL Constraints overview.