Why NULL in AND, OR, NOT logic in SQL? - Purpose & Use Cases

The Big Idea

What if your queries silently miss important data just because you misunderstood NULL?

The Scenario

Imagine you have a list of tasks with some missing information about their status. You want to find tasks that are both urgent and completed. Without understanding how missing data (NULL) behaves, you might try to check both conditions manually.

The Problem

Manually checking each task's status and urgency is slow and confusing because NULL means 'unknown'. If you treat NULL as true or false without care, you get wrong results or miss tasks. This makes your filtering unreliable and error-prone.

The Solution

Understanding how NULL works with AND, OR, and NOT lets you write queries that correctly handle unknown values. SQL uses three-valued logic to decide if a condition is true, false, or unknown, so your filters work as expected even with missing data.

Before vs After

✗ Before

SELECT * FROM tasks WHERE urgent = TRUE AND completed = TRUE;

✓ After

SELECT * FROM tasks WHERE urgent = TRUE AND completed IS TRUE;

What It Enables

You can confidently filter and combine conditions even when some data is missing, making your queries accurate and trustworthy.

Real Life Example

A project manager wants to see all tasks that are urgent and definitely completed. Some tasks have no completion info yet (NULL). Using proper NULL logic, the manager avoids showing tasks with unknown completion status.

Key Takeaways

NULL means unknown, not true or false.

AND, OR, NOT with NULL follow special three-valued logic.

Proper handling of NULL prevents wrong query results.