NULL in AND, OR, NOT logic in SQL

Introduction

NULL means unknown in databases. Understanding how NULL works with AND, OR, and NOT helps you get correct answers when data is missing or unknown.

When you want to check if multiple conditions are true but some values might be missing.

When filtering data that may have unknown or missing values.

When combining conditions and you need to know how unknown values affect the result.

When writing queries that use NOT to exclude certain rows but some values are NULL.

When debugging why some rows appear or disappear unexpectedly in query results.

Syntax

SQL

condition1 AND condition2
condition1 OR condition2
NOT condition

NULL means unknown, so logical operations with NULL can result in NULL (unknown) instead of TRUE or FALSE.

In SQL, TRUE, FALSE, and NULL form a three-valued logic system.

Examples

Result is NULL because one side is unknown.

SQL

TRUE AND NULL

Result is NULL because one side is unknown and the other is false.

SQL

FALSE OR NULL

Result is NULL because NOT unknown is still unknown.

SQL

NOT NULL

Result is TRUE because OR with TRUE always returns TRUE.

SQL

TRUE OR NULL

Sample Program

This query shows how NULL behaves with AND, OR, and NOT.

SQL

SELECT
  TRUE AND NULL AS and_result,
  FALSE OR NULL AS or_result,
  NOT NULL AS not_result,
  TRUE OR NULL AS or_true_result;

Important Notes

When NULL is involved, AND returns TRUE only if both sides are TRUE.

OR returns TRUE if any side is TRUE, even if the other is NULL.

NOT NULL remains NULL because negation of unknown is still unknown.

Summary

NULL means unknown and affects logical operations differently than TRUE or FALSE.

AND with NULL returns NULL unless the other side is FALSE (then FALSE).

OR with NULL returns TRUE if the other side is TRUE, else NULL.

NOT NULL always returns NULL.