IS NULL vs equals NULL in SQL - Interactive Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to select rows where the column 'age' has no value.

SQL

SELECT * FROM users WHERE age [1];

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

A= NULL

BIS NOT NULL

C= 0

DIS NULL

Attempts:

3 left

💡 Hint

Common Mistakes

Using '= NULL' instead of 'IS NULL' causes no rows to be returned.

Confusing NULL with zero or empty string.

In SQL, to check if a column has no value (NULL), you must use IS NULL. Using = NULL does not work as expected.

2fill in blank

medium

Complete the code to select rows where the column 'email' is not missing.

SQL

SELECT * FROM contacts WHERE email [1];

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

AIS NOT NULL

B= NULL

C= ''

DIS NULL

Attempts:

3 left

💡 Hint

Common Mistakes

Using '= NULL' which never matches any row.

Using '= ''' which checks for empty string, not NULL.

To find rows where 'email' has a value, use IS NOT NULL. This excludes rows where 'email' is NULL.

3fill in blank

hard

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

SQL

SELECT * FROM results WHERE score [1];

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

A!= NULL

B= NULL

CIS NULL

D= 0

Attempts:

3 left

💡 Hint

Common Mistakes

Using '= NULL' or '!= NULL' which always return false.

Confusing NULL with zero or other values.

Using = NULL or != NULL does not work in SQL. Use IS NULL to check for missing values.

4fill in blank

hard

Fill both blanks to select rows where 'last_login' is missing and 'status' is active.

SQL

SELECT * FROM users WHERE last_login [1] AND status = [2];

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

AIS NULL

B'active'

C'inactive'

D= NULL

Attempts:

3 left

💡 Hint

Common Mistakes

Using '= NULL' instead of 'IS NULL'.

Forgetting quotes around string values.

Use IS NULL to check for missing 'last_login'. For 'status', use the string value 'active' to filter active users.

5fill in blank

hard

Fill all three blanks to select rows where 'comment' is not missing, 'rating' is greater than 3, and 'verified' is true.

SQL

SELECT * FROM feedback WHERE comment [1] AND rating [2] 3 AND verified = [3];

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

AIS NOT NULL

B>

CTRUE

DIS NULL

Attempts:

3 left

💡 Hint

Common Mistakes

Using '= NULL' or '!= NULL' for NULL checks.

Using '=' instead of '>' for rating comparison.

Using 1 or 'true' (string) instead of TRUE (boolean).

Use IS NOT NULL to find rows where 'comment' exists. Use > to check 'rating' greater than 3. Use TRUE for boolean 'verified'.

Practice

1. Which SQL expression correctly checks if a column age has missing (NULL) values?

easy

A. age IS NULL

B. age = NULL

C. age == NULL

D. age != NULL

Solution

1. Step 1: Understand NULL in SQL

   NULL means unknown or missing data, and it cannot be compared with = or != operators.

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

   The correct way to check for NULL is using IS NULL, which returns true for missing values.

3. Final Answer:

   age IS NULL -> Option A

4. Quick Check:

   Use IS NULL to check missing values [OK]

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

Common Mistakes:

• Using = NULL returns no rows
• Confusing IS NULL with equality operator
• Using == NULL which is invalid in SQL

2. Which of the following SQL WHERE clauses is syntactically correct to find rows where salary is NULL?

easy

A. WHERE salary IS NULL

B. WHERE salary = NULL

C. WHERE salary == NULL

D. WHERE salary != NULL

Solution

1. Step 1: Review SQL syntax for NULL checks

   SQL uses IS NULL to check for NULL values; equality operators (=, ==) are invalid here.

2. Step 2: Identify the correct syntax

   Only WHERE salary IS NULL is valid and will run without error.

3. Final Answer:

   WHERE salary IS NULL -> Option A

4. Quick Check:

   IS NULL is correct syntax for NULL checks [OK]

Hint: IS NULL is the only valid syntax for NULL checks [OK]

Common Mistakes:

• Using = NULL causes syntax errors or no matches
• Using == NULL is invalid SQL
• Using != NULL does not find NULLs

3. Given the table employees with column manager_id that can be NULL, what will this query return?

SELECT * FROM employees WHERE manager_id = NULL;

medium

A. All rows where manager_id is NULL

B. Syntax error

C. No rows

D. All rows where manager_id is not NULL

Solution

1. Step 1: Understand comparison with NULL using = operator

   In SQL, = NULL does not match any rows because NULL is unknown and cannot be compared with =.

2. Step 2: Result of the query

   The query returns zero rows because no value equals NULL using = operator.

3. Final Answer:

   No rows -> Option C

4. Quick Check:

   = NULL returns no rows [OK]

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

Common Mistakes:

• Expecting = NULL to find NULL values
• Confusing NULL with empty string or zero
• Assuming syntax error instead of empty result

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

SELECT * FROM users WHERE email = NULL;

But it returns no rows. What is the fix?

medium

A. Use email IS NOT NULL

B. Change to email != NULL

C. Add quotes: email = 'NULL'

D. Change to email IS NULL

Solution

1. Step 1: Identify incorrect NULL comparison

   Using = NULL does not find NULL values because NULL is not equal to anything.

2. Step 2: Correct NULL check syntax

   Use IS NULL to properly find rows where email is missing.

3. Final Answer:

   Change to email IS NULL -> Option D

4. Quick Check:

   Use IS NULL to find missing values [OK]

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

Common Mistakes:

• Using != NULL expecting to find NULLs
• Using string 'NULL' instead of NULL keyword
• Using IS NOT NULL which finds non-missing values

5. You want to select all customers who have no recorded phone number or whose phone number is an empty string. Which WHERE clause correctly finds these rows?

SELECT * FROM customers WHERE ???

hard

A. phone = NULL OR phone = ''

B. phone IS NULL OR phone = ''

C. phone IS NULL AND phone = ''

D. phone = NULL AND phone = ''

Solution

1. Step 1: Understand NULL and empty string difference

   NULL means missing data, empty string means data is present but empty.

2. Step 2: Combine conditions correctly

   Use phone IS NULL to find missing phone numbers and phone = '' to find empty strings, combined with OR to get either case.

3. Final Answer:

   phone IS NULL OR phone = '' -> Option B

4. Quick Check:

   Use IS NULL for missing, = '' for empty [OK]

Hint: Use IS NULL OR = '' to find missing or empty values [OK]

Common Mistakes:

• Using = NULL which never matches
• Using AND instead of OR excludes all rows
• Confusing empty string with NULL