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.

Practice

1. What is the result of the SQL expression TRUE AND NULL?

easy

A. TRUE

B. NULL

C. FALSE

D. Error

Solution

1. Step 1: Understand AND with NULL

   In SQL, AND returns FALSE if any side is FALSE. If one side is TRUE and the other is NULL, the result is NULL because the unknown value affects the outcome.

2. Step 2: Apply to TRUE AND NULL

   Since the first operand is TRUE and the second is NULL, the result is NULL.

3. Final Answer:

   NULL -> Option B

4. Quick Check:

   TRUE AND NULL = NULL [OK]

Hint: AND with NULL returns NULL unless FALSE present [OK]

Common Mistakes:

• Assuming TRUE AND NULL is TRUE
• Confusing NULL with FALSE
• Thinking NULL causes error

2. Which of the following is the correct SQL syntax to check if a column col is NOT NULL?

easy

A. col != NULL

B. col <> NULL

C. col IS NOT NULL

D. NOT col = NULL

Solution

1. Step 1: Recall NULL comparison rules

   In SQL, you cannot use = or != to compare with NULL. Instead, use IS NULL or IS NOT NULL.

2. Step 2: Identify correct syntax

   The correct way to check if col is not null is col IS NOT NULL.

3. Final Answer:

   col IS NOT NULL -> Option C

4. Quick Check:

   Use IS NOT NULL for null checks [OK]

Hint: Use IS NOT NULL, not != or <> [OK]

Common Mistakes:

• Using != or <> to compare with NULL
• Using NOT col = NULL which is invalid
• Confusing NULL with empty string

3. Given a table Test with a column flag that can be TRUE, FALSE, or NULL, what is the result of this query?

SELECT * FROM Test WHERE flag OR FALSE;

medium

A. Rows where flag is TRUE

B. Rows where flag is TRUE or NULL

C. Rows where flag is TRUE or FALSE

D. No rows returned

Solution

1. Step 1: Understand OR with FALSE

   In SQL, OR FALSE returns the value of the first operand because FALSE does not affect the OR result.

2. Step 2: Evaluate flag OR FALSE

   If flag is TRUE, the expression is TRUE. If flag is FALSE, expression is FALSE. If flag is NULL, expression is NULL (unknown), which is treated as FALSE in WHERE filtering.

3. Final Answer:

   Rows where flag is TRUE -> Option A

4. Quick Check:

   flag OR FALSE = flag; WHERE filters TRUE only [OK]

Hint: OR FALSE returns first operand value [OK]

Common Mistakes:

• Including NULL rows in result
• Thinking OR FALSE always returns TRUE
• Confusing OR with AND behavior

4. Consider this SQL WHERE clause:

WHERE NOT (col = NULL)

What is the problem with this condition?

medium

A. It always returns NULL (no rows match)

B. It always returns TRUE

C. It always returns FALSE

D. It causes a syntax error

Solution

1. Step 1: Understand comparison with NULL

   In SQL, col = NULL always returns NULL (unknown), never TRUE or FALSE.

2. Step 2: Apply NOT to NULL

   NOT NULL is still NULL, so the condition never evaluates to TRUE, filtering out all rows.

3. Final Answer:

   It always returns NULL (no rows match) -> Option A

4. Quick Check:

   NOT (col = NULL) = NULL, no matches [OK]

Hint: NOT NULL remains NULL, no rows pass filter [OK]

Common Mistakes:

• Expecting NOT NULL to be TRUE
• Using = NULL instead of IS NULL
• Assuming syntax error occurs

5. You want to select rows from a table Employees where the ManagerID is either NULL or the employee is active (Active = TRUE). Which WHERE clause correctly handles NULL logic?

hard

A. WHERE ManagerID IS NOT NULL OR Active = TRUE

B. WHERE ManagerID = NULL OR Active = TRUE

C. WHERE NOT ManagerID = NULL AND Active = TRUE

D. WHERE ManagerID IS NULL OR Active = TRUE

Solution

1. Step 1: Correctly check for NULL

   Use IS NULL to check if ManagerID is NULL. Using = NULL is invalid and returns NULL.

2. Step 2: Combine conditions with OR

   The condition requires rows where ManagerID is NULL or Active is TRUE, so use OR between these checks.

3. Final Answer:

   WHERE ManagerID IS NULL OR Active = TRUE -> Option D

4. Quick Check:

   Use IS NULL and OR for correct logic [OK]

Hint: Use IS NULL, combine with OR for either condition [OK]

Common Mistakes:

• Using = NULL instead of IS NULL
• Using AND instead of OR
• Using IS NOT NULL which is opposite