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.

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