NULL behavior in comparisons in SQL - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ query_result

intermediate

Output of NULL comparison in WHERE clause

Consider a table Employees with a column ManagerID that can be NULL. What rows will this query return?

SELECT * FROM Employees WHERE ManagerID = NULL;

SQL

SELECT * FROM Employees WHERE ManagerID = NULL;

A All rows are returned regardless of ManagerID value.

B All rows where ManagerID is NULL are returned.

C No rows are returned because NULL = NULL is unknown.

D The query causes a syntax error.

💡 Hint

Remember that NULL means unknown, so comparisons with NULL don't behave like normal values.

In SQL, comparing any value to NULL using = returns unknown, which is treated as false in WHERE clauses. To check for NULL, use IS NULL.

❓ query_result

intermediate

Effect of IS NULL vs = NULL in filtering

Given a table Orders with a column ShippedDate that can be NULL, which query returns all orders that have not been shipped?

1) SELECT * FROM Orders WHERE ShippedDate = NULL;
2) SELECT * FROM Orders WHERE ShippedDate IS NULL;

SQL

SELECT * FROM Orders WHERE ShippedDate IS NULL;

A Neither query returns any rows.

B Only query 1 returns the correct rows.

C Both queries return the same rows.

D Only query 2 returns the correct rows.

💡 Hint

Think about how SQL treats NULL in equality comparisons versus IS NULL.

The = NULL comparison always returns unknown, so query 1 returns no rows. Query 2 uses IS NULL which correctly filters rows with NULL values.

🧠 Conceptual

advanced

Understanding three-valued logic with NULL

In SQL, what is the result of the expression (NULL <> 5) in a WHERE clause filter?

A TRUE

B UNKNOWN (treated as FALSE in WHERE)

C FALSE

D Syntax error

💡 Hint

Remember SQL uses three-valued logic: TRUE, FALSE, and UNKNOWN.

Any comparison with NULL results in UNKNOWN, which acts like FALSE in WHERE filters, so the row is not selected.

📝 Syntax

advanced

Correct syntax to check for NULL in SQL

Which of the following SQL WHERE clauses correctly filters rows where the column Price is NULL?

A WHERE Price IS NULL

B WHERE Price == NULL

C WHERE Price = NULL

D WHERE Price != NULL

💡 Hint

Think about the special keyword used to check NULL values.

The correct syntax to check for NULL is using IS NULL. Using = NULL or != NULL is invalid or returns unknown.

❓ optimization

expert

Optimizing queries with NULL checks

You want to find all customers who have not placed any orders. The Orders table has a nullable CustomerID column. Which query is most efficient and correct?

A) SELECT * FROM Customers WHERE CustomerID NOT IN (SELECT CustomerID FROM Orders);
B) SELECT * FROM Customers WHERE CustomerID NOT IN (SELECT CustomerID FROM Orders WHERE CustomerID IS NOT NULL);
C) SELECT * FROM Customers WHERE CustomerID NOT EXISTS (SELECT 1 FROM Orders WHERE Orders.CustomerID = Customers.CustomerID);
D) SELECT * FROM Customers WHERE CustomerID IS NULL;

A Option C

B Option A

C Option D

D Option B

💡 Hint

Consider how NULLs affect NOT IN and EXISTS clauses.

NOT IN with NULLs in subquery can cause no rows to be returned due to unknown comparisons. Using NOT EXISTS avoids this problem and is more efficient.

Practice

1. What does NULL represent in a database?

easy

A. False boolean

B. Zero value

C. Empty string

D. Missing or unknown data

Solution

1. Step 1: Understand the meaning of NULL

   NULL in databases means the data is missing or unknown, not zero or empty.

2. Step 2: Differentiate NULL from other values

   Zero, empty string, and false are actual values, while NULL means no value is stored.

3. Final Answer:

   Missing or unknown data -> Option D

4. Quick Check:

   NULL means missing or unknown data [OK]

Hint: NULL means unknown or missing, not zero or empty [OK]

Common Mistakes:

• Confusing NULL with zero
• Thinking NULL is an empty string
• Assuming NULL means false

2. Which of the following is the correct way to check if a column col is NULL in SQL?

easy

A. WHERE col = NULL

B. WHERE col == NULL

C. WHERE col IS NULL

D. WHERE col <> NULL

Solution

1. Step 1: Recall SQL syntax for NULL checks

   In SQL, to check for NULL, you must use IS NULL because = NULL does not work.

2. Step 2: Eliminate incorrect syntax

   col = NULL and col == NULL are invalid for NULL checks; col <> NULL also does not work.

3. Final Answer:

   WHERE col IS NULL -> Option C

4. Quick Check:

   Use IS NULL to check NULL values [OK]

Hint: Use IS NULL, not = or ==, to check for NULL [OK]

Common Mistakes:

• Using = NULL instead of IS NULL
• Using == NULL which is invalid in SQL
• Using <> NULL to check for NULL

3. Given a table users with a column age that contains some NULL values, what will this query return?

SELECT * FROM users WHERE age <> 30;

medium

A. All rows where age is not 30, including rows with NULL age

B. Only rows where age is not 30 and not NULL

C. Only rows where age is 30

D. No rows, because NULL causes error

Solution

1. Step 1: Understand comparison with NULL

   Comparisons like age <> 30 return UNKNOWN for NULL values, so those rows are excluded.

2. Step 2: Determine which rows are returned

   Only rows where age is not 30 and age is not NULL will be returned.

3. Final Answer:

   Only rows where age is not 30 and not NULL -> Option B

4. Quick Check:

   Comparisons with NULL exclude NULL rows [OK]

Hint: Comparisons exclude NULL rows, so NULLs are ignored [OK]

Common Mistakes:

• Assuming NULL rows are included
• Thinking NULL causes errors in comparisons
• Confusing <> with IS NOT NULL

4. You wrote this query to find rows where status is not NULL:

SELECT * FROM orders WHERE status <> NULL;

Why does it return no rows, and how to fix it?

medium

A. Comparison with NULL returns UNKNOWN; use status IS NOT NULL

B. Syntax error; use status != NULL

C. NULL is treated as zero; use status <> 0

D. Query is correct; no rows have status

Solution

1. Step 1: Understand NULL comparison behavior

   Comparing with NULL using <> returns UNKNOWN, so no rows match.

2. Step 2: Use correct syntax to check NOT NULL

   Use status IS NOT NULL to find rows where status is not NULL.

3. Final Answer:

   Comparison with NULL returns UNKNOWN; use status IS NOT NULL -> Option A

4. Quick Check:

   Use IS NOT NULL to check for non-NULL values [OK]

Hint: Use IS NOT NULL, not <> NULL, to find non-NULL rows [OK]

Common Mistakes:

• Using <> NULL instead of IS NOT NULL
• Assuming <> NULL works like != NULL
• Thinking NULL is zero or empty

5. You want to select all rows from products where discount is either NULL or greater than 10. Which query correctly returns these rows?

hard

A. SELECT * FROM products WHERE discount > 10 OR discount IS NULL;

B. SELECT * FROM products WHERE discount > 10 AND discount IS NULL;

C. SELECT * FROM products WHERE discount > 10 OR discount = NULL;

D. SELECT * FROM products WHERE discount > 10 AND discount = NULL;

Solution

1. Step 1: Understand NULL checks with OR condition

   To include NULL values, use IS NULL, not = NULL.

2. Step 2: Combine conditions correctly

   Use OR to select rows where discount is greater than 10 or discount is NULL.

3. Final Answer:

   SELECT * FROM products WHERE discount > 10 OR discount IS NULL; -> Option A

4. Quick Check:

   Use IS NULL with OR to include NULLs [OK]

Hint: Use OR with IS NULL to include NULL values in conditions [OK]

Common Mistakes:

• Using = NULL instead of IS NULL
• Using AND instead of OR with NULL checks
• Assuming NULL compares like normal values