NULL in AND, OR, NOT logic in SQL - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ query_result

intermediate

Output of AND with NULL

What is the result of the following SQL expression?

SELECT TRUE AND NULL AS result;

SQL

SELECT TRUE AND NULL AS result;

A Syntax Error

B TRUE

C FALSE

D NULL

💡 Hint

Remember that NULL means unknown in SQL logic.

In SQL, TRUE AND NULL results in NULL because the truth value is unknown when combined with TRUE.

❓ query_result

intermediate

Output of OR with NULL

What is the result of this SQL expression?

SELECT FALSE OR NULL AS result;

SQL

SELECT FALSE OR NULL AS result;

A TRUE

B NULL

C FALSE

D Runtime Error

💡 Hint

Think about how OR behaves when one operand is unknown.

FALSE OR NULL results in NULL because the overall truth is unknown when combined with FALSE and NULL.

❓ query_result

advanced

Output of NOT with NULL

What is the result of this SQL expression?

SELECT NOT NULL AS result;

SQL

SELECT NOT NULL AS result;

A Syntax Error

B FALSE

C NULL

D TRUE

💡 Hint

NOT reverses TRUE to FALSE and FALSE to TRUE, but what about NULL?

NOT NULL results in NULL because negation of unknown remains unknown in SQL.

🧠 Conceptual

advanced

Understanding NULL in complex AND/OR expressions

Consider the expression:

SELECT (TRUE OR NULL) AND FALSE AS result;

What is the output?

SQL

SELECT (TRUE OR NULL) AND FALSE AS result;

A FALSE

B TRUE

C NULL

D Syntax Error

💡 Hint

Evaluate the OR part first, then AND with FALSE.

TRUE OR NULL is TRUE because TRUE dominates OR. Then TRUE AND FALSE is FALSE.

🧠 Conceptual

expert

Behavior of NULL in NOT combined with AND and OR

What is the result of this SQL expression?

SELECT NOT (FALSE AND NULL) OR (NULL AND TRUE) AS result;

SQL

SELECT NOT (FALSE AND NULL) OR (NULL AND TRUE) AS result;

A TRUE

B FALSE

C NULL

D Runtime Error

💡 Hint

Evaluate inside parentheses first, then apply NOT and OR.

FALSE AND NULL is FALSE (because FALSE AND anything is FALSE). NOT FALSE is TRUE. NULL AND TRUE is NULL. TRUE OR NULL is TRUE.