Why equals NULL fails in SQL - Test Your Understanding

Practice - 5 Tasks

1fill in blank

easy

Complete the code to check if 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!=

CIS

D<>

Attempts:

3 left

💡 Hint

Common Mistakes

Using '=' to compare with NULL

Using '<>' to compare with NULL

In SQL, to check if a value is NULL, you must use IS NULL. Using = NULL does not work because NULL means unknown.

2fill in blank

medium

Complete the code to find rows where 'score' is not NULL.

SQL

SELECT * FROM results WHERE score [1] NULL;

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

AIS NOT

B=

C!=

D<>

Attempts:

3 left

💡 Hint

Common Mistakes

Using '!=' or '<>' to check for NOT NULL

Using '=' instead of 'IS NOT'

To check for values that are not NULL, use IS NOT NULL. Other operators do not work with NULL.

3fill in blank

hard

Fix the error in the WHERE clause to correctly filter NULL values.

SQL

SELECT * FROM orders WHERE delivery_date [1] NULL;

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

A!=

BIS

C=

DLIKE

Attempts:

3 left

💡 Hint

Common Mistakes

Using '=' or '!=' with NULL

Using 'LIKE' with NULL

Use IS NULL to check for NULL values. Operators like '=' or '!=' do not work with NULL.

4fill in blank

hard

Fill both blanks to correctly filter rows where 'status' is NULL or not NULL.

SQL

SELECT * FROM tasks WHERE status [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 '!=' instead of IS or IS NOT

Mixing up IS and IS NOT

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

5fill in blank

hard

Fill all three blanks to create a query that selects rows where 'comment' is NULL, 'rating' is not NULL, and 'approved' equals 1.

SQL

SELECT * FROM feedback WHERE comment [1] NULL AND rating [2] NULL AND approved [3] 1;

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

AIS

BIS NOT

C=

D!=

Attempts:

3 left

💡 Hint

Common Mistakes

Using '=' or '!=' to compare with NULL

Using IS or IS NOT for normal values

Use IS NULL to check for NULL, IS NOT NULL for not NULL, and = for normal value comparison.

Practice

1. Why does the SQL condition column = NULL always fail to find rows with NULL values?

easy

A. Because NULL means unknown and cannot be compared with = or != operators

B. Because NULL is treated as zero in SQL

C. Because NULL is a string and needs quotes

D. Because NULL values are automatically excluded from queries

Solution

1. Step 1: Understand NULL meaning in SQL

   NULL represents an unknown or missing value, not a specific value.

2. Step 2: Explain comparison behavior with NULL

   Comparing anything to NULL with = or != always returns false because NULL is unknown.

3. Final Answer:

   Because NULL means unknown and cannot be compared with = or != operators -> Option A

4. Quick Check:

   NULL comparison with = fails [OK]

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

Common Mistakes:

• Using = NULL to find NULL values
• Assuming NULL equals zero
• Trying to quote NULL as a string

2. Which of the following SQL WHERE clauses correctly checks for NULL values in the age column?

easy

A. WHERE age != NULL

B. WHERE age = NULL

C. WHERE age IS NULL

D. WHERE age = 'NULL'

Solution

1. Step 1: Review correct syntax to check NULL

   SQL uses IS NULL to test if a column has NULL values.

2. Step 2: Analyze each option

   WHERE age = NULL and C use = or != with NULL which always fail; WHERE age = 'NULL' treats NULL as a string which is incorrect.

3. Final Answer:

   WHERE age IS NULL -> Option C

4. Quick Check:

   Use IS NULL for NULL checks [OK]

Hint: Always use IS NULL or IS NOT NULL for NULL checks [OK]

Common Mistakes:

• Using = NULL or != NULL in WHERE clause
• Quoting NULL as a string
• Confusing NULL with empty string

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

SELECT * FROM users WHERE email = NULL;

medium

A. All rows where email is NULL

B. All rows where email is not NULL

C. Syntax error

D. No rows at all

Solution

1. Step 1: Understand the effect of = NULL in WHERE clause

   Comparing with = NULL always yields unknown (false), so no rows match.

2. Step 2: Predict query result

   The query returns zero rows because no row satisfies email = NULL.

3. Final Answer:

   No rows at all -> Option D

4. Quick Check:

   email = NULL returns no rows [OK]

Hint: = NULL never matches any row, returns empty result [OK]

Common Mistakes:

• Expecting rows with NULL to be returned
• Thinking = NULL is valid syntax
• Confusing NULL with empty string

4. You wrote this query to find rows with NULL in salary:

SELECT * FROM employees WHERE salary = NULL;

But it returns no rows. What is the fix?

medium

A. Use WHERE salary != NULL

B. Change to WHERE salary IS NULL

C. Add quotes: WHERE salary = 'NULL'

D. Change to WHERE salary = 0

Solution

1. Step 1: Identify why salary = NULL fails

   Comparing with = NULL always returns false, so no rows match.

2. Step 2: Use correct syntax to check NULL

   Use IS NULL to find rows where salary is NULL.

3. Final Answer:

   Change to WHERE salary IS NULL -> Option B

4. Quick Check:

   Use IS NULL, not = NULL [OK]

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

Common Mistakes:

• Using = NULL instead of IS NULL
• Comparing NULL to zero
• Quoting NULL as string

5. You want to select all rows from orders where shipped_date is either NULL or equals '2024-01-01'. Which WHERE clause is correct?

hard

A. WHERE shipped_date IS NULL OR shipped_date = '2024-01-01'

B. WHERE shipped_date = NULL OR shipped_date = '2024-01-01'

C. WHERE shipped_date = 'NULL' OR shipped_date = '2024-01-01'

D. WHERE shipped_date != NULL OR shipped_date = '2024-01-01'

Solution

1. Step 1: Understand NULL comparison rules

   Use IS NULL to check for NULL values, not = NULL or != NULL.

2. Step 2: Analyze the WHERE clause options

   WHERE shipped_date IS NULL OR shipped_date = '2024-01-01' correctly uses IS NULL and equals comparison; others misuse NULL comparisons or treat NULL as string.

3. Final Answer:

   WHERE shipped_date IS NULL OR shipped_date = '2024-01-01' -> Option A

4. Quick Check:

   Combine IS NULL with = for dates [OK]

Hint: Use IS NULL for NULL, = for values in OR conditions [OK]

Common Mistakes:

• Using = NULL or != NULL in OR conditions
• Quoting NULL as string
• Mixing NULL checks with wrong operators