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

Practice - 5 Tasks

1fill in blank

easy

Complete the code to select rows where the column 'age' is NULL.

SQL

SELECT * FROM users WHERE age [1] NULL;

Drag options to blanks, or click blank then click option'

A<>

B!=

C=

DIS

Attempts:

3 left

💡 Hint

Common Mistakes

Using '=' to compare with NULL, which always returns false.

In SQL, NULL is not a value, so you must use IS or IS NOT to check for NULL.

2fill in blank

medium

Complete the code to select rows where 'salary' is NOT NULL.

SQL

SELECT * FROM employees WHERE salary [1] NULL;

Drag options to blanks, or click blank then click option'

A=

B<>

CIS NOT

D!=

Attempts:

3 left

💡 Hint

Common Mistakes

Using '!=' or '<>' to check for NOT NULL, which does not work correctly.

To check that a column is not NULL, use IS NOT NULL in SQL.

3fill in blank

hard

Fix the error in the query to find rows where 'score' is NULL.

SQL

SELECT * FROM results WHERE score [1] NULL;

Drag options to blanks, or click blank then click option'

ALIKE

BIS

C=

DIN

Attempts:

3 left

💡 Hint

Common Mistakes

Using '=' or 'LIKE' to compare with NULL.

Use IS NULL to check for NULL values, not '=' or other operators.

4fill in blank

hard

Fill both blanks to select rows where 'date' is NULL or 'status' is NOT NULL.

SQL

SELECT * FROM orders WHERE date [1] NULL OR status [2] NULL;

Drag options to blanks, or click blank then click option'

AIS

BIS NOT

C=

D!=

Attempts:

3 left

💡 Hint

Common Mistakes

Using '=' or '!=' to check for NULL or NOT NULL.

Use IS NULL to check for NULL and IS NOT NULL to check for NOT NULL.

5fill in blank

hard

Fill all three blanks to select rows where 'price' is NULL, 'quantity' is NOT NULL, and 'discount' equals 0.

SQL

SELECT * FROM sales WHERE price [1] NULL AND quantity [2] NULL AND discount [3] 0;

Drag options to blanks, or click blank then click option'

AIS

BIS NOT

C=

D!=

Attempts:

3 left

💡 Hint

Common Mistakes

Using '=' to check for NULL or '!=' to check for NOT NULL.

Use IS NULL for NULL, IS NOT NULL for NOT NULL, and '=' for exact value comparison.

Practice

1. What does NULL represent in a database?

easy

A. Missing or unknown data

B. Zero value

C. Empty string

D. Boolean false

Solution

1. Step 1: Understand the meaning of NULL

   NULL means that the data is missing or unknown, not a specific value like zero or empty string.

2. Step 2: Differentiate NULL from other values

   Zero, empty string, and false are actual values, but NULL means no value is present.

3. Final Answer:

   Missing or unknown data -> Option A

4. Quick Check:

   NULL means missing data = C [OK]

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

Common Mistakes:

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

2. Which SQL syntax correctly checks if a column age is NULL?

easy

A. WHERE age = NULL

B. WHERE age == NULL

C. WHERE age IS NULL

D. WHERE age != NULL

Solution

1. Step 1: Recall correct NULL comparison syntax

   In SQL, you cannot use = or != to check for NULL because NULL is not a value.

2. Step 2: Use IS NULL to check for NULL

   The correct way is to use IS NULL to find rows where the column is NULL.

3. Final Answer:

   WHERE age IS NULL -> Option C

4. Quick Check:

   Use IS NULL, not = NULL [OK]

Hint: Use IS NULL, never = NULL to check for NULL [OK]

Common Mistakes:

• Using = NULL instead of IS NULL
• Using == NULL which is invalid in SQL
• Using != NULL which does not work

3. Given a table users with column email, what will this query return?

SELECT * FROM users WHERE email = NULL;

medium

A. All rows where email is NULL

B. No rows

C. Syntax error

D. All rows

Solution

1. Step 1: Understand comparison with NULL using =

   Using = NULL does not work because NULL is not a value and comparisons with NULL return unknown.

2. Step 2: Result of WHERE email = NULL

   The condition email = NULL evaluates to unknown for all rows, so no rows match and none are returned.

3. Final Answer:

   No rows -> Option B

4. Quick Check:

   email = NULL returns no rows [OK]

Hint: = NULL never matches; use IS NULL instead [OK]

Common Mistakes:

• Expecting rows with NULL to be returned
• Thinking = NULL works like IS NULL
• Confusing NULL with empty string

4. You want to find rows where salary is NULL, but this query returns no rows:

SELECT * FROM employees WHERE salary = NULL;

What is the error and how to fix it?

medium

A. Use IS NULL instead of = NULL

B. Use salary = 0 instead of NULL

C. Use salary != NULL instead

D. Add quotes around NULL

Solution

1. Step 1: Identify incorrect NULL comparison

   The query uses = NULL which does not work because NULL is not a value and cannot be compared with =.

2. Step 2: Correct syntax to find NULL values

   Use IS NULL to check for NULL values in SQL.

3. Final Answer:

   Use IS NULL instead of = NULL -> Option A

4. Quick Check:

   IS NULL finds NULLs, = NULL does not [OK]

Hint: Replace = NULL with IS NULL to fix query [OK]

Common Mistakes:

• Using = NULL instead of IS NULL
• Trying != NULL which also fails
• Using 0 or empty string instead of NULL

5. You want to count how many rows have a missing phone_number in table contacts. Which query correctly counts NULLs?

hard

A. SELECT COUNT(phone_number) FROM contacts WHERE phone_number IS NULL;

B. SELECT COUNT(phone_number) FROM contacts;

C. SELECT COUNT(*) FROM contacts WHERE phone_number = NULL;

D. SELECT COUNT(*) FROM contacts WHERE phone_number IS NULL;

Solution

1. Step 1: Understand counting NULLs

   COUNT(column) counts only non-NULL values, so it ignores NULLs.

2. Step 2: Use WHERE phone_number IS NULL to filter NULL rows

   To count rows with NULL phone_number, filter with IS NULL and count all rows with COUNT(*)

3. Final Answer:

   SELECT COUNT(*) FROM contacts WHERE phone_number IS NULL; -> Option D

4. Quick Check:

   Use IS NULL filter and COUNT(*) to count NULL rows [OK]

Hint: Filter with IS NULL and use COUNT(*) to count NULLs [OK]

Common Mistakes:

• Using = NULL in WHERE clause
• Using COUNT(column) which skips NULLs
• Filtering with COUNT(phone_number) which is invalid