Why NULL is not a value in SQL - Challenge Your Understanding

Challenge - 5 Problems

🧠 Conceptual

intermediate

Understanding NULL in SQL

Why is NULL considered not a value in SQL?

A Because NULL is the same as the number zero.

B Because NULL is treated as zero in all calculations.

C Because NULL means the data is missing or unknown, not a specific value.

D Because NULL is a string that represents empty text.

💡 Hint

Think about what NULL represents in a database column when no data is entered.

NULL means the data is missing or unknown. It is not a value like zero or empty string. It represents absence of any value.

❓ query_result

intermediate

Query Result with NULL Comparison

What is the result of this SQL query?

SELECT 1 = NULL AS result;

A NULL

B FALSE

C TRUE

D Syntax Error

💡 Hint

Remember how NULL behaves in comparisons.

Any comparison with NULL returns NULL (unknown), not TRUE or FALSE.

📝 Syntax

advanced

Correct NULL Check in WHERE Clause

Which SQL WHERE clause correctly checks if a column col is NULL?

A WHERE col = NULL

B WHERE col IS NULL

C WHERE col == NULL

D WHERE col != NULL

💡 Hint

Think about how NULL comparisons differ from normal values.

To check for NULL, use IS NULL. Using = NULL or == NULL does not work.

❓ optimization

advanced

Optimizing NULL Checks in SQL Queries

Which option is the most efficient way to filter rows where col is NOT NULL?

A WHERE col IS NOT NULL

B WHERE NOT col = NULL

C WHERE col <> NULL

D WHERE col != NULL

💡 Hint

Consider which syntax is standard and optimized for NULL checks.

IS NOT NULL is the standard and optimized way to check for non-NULL values.

🔧 Debug

expert

Why does this query return no rows?

Given a table users with some NULL values in age, why does this query return no rows?

SELECT * FROM users WHERE age <> NULL;

A Because NULL is treated as zero and no ages are zero.

B Because the table is empty.

C Because the syntax is invalid and causes an error.

D Because <code>age <> NULL</code> always evaluates to NULL, so no rows match.

💡 Hint

Think about how NULL behaves in inequality comparisons.

Comparisons with NULL return NULL (unknown), so age <> NULL never matches any row.