IS NULL and IS NOT NULL in MySQL - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to select all rows where the column 'email' has no value (is NULL).

MySQL

SELECT * FROM users WHERE email [1];

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

A= NULL

BIS NULL

CIS NOT NULL

D!= NULL

Attempts:

3 left

💡 Hint

Common Mistakes

Using '= NULL' instead of 'IS NULL'

Using '!=' or '<>' to check for NULL

To check if a column has no value, use IS NULL. Using = NULL does not work in SQL.

2fill in blank

medium

Complete the code to select all rows where the column 'phone' has a value (is NOT NULL).

MySQL

SELECT * FROM contacts WHERE phone [1];

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

AIS NOT NULL

B!= NULL

CIS NULL

D= NULL

Attempts:

3 left

💡 Hint

Common Mistakes

Using '= NULL' or '!= NULL' which do not work for NULL checks

Using 'IS NULL' instead of 'IS NOT NULL'

To find rows where a column has a value, use IS NOT NULL.

3fill in blank

hard

Fix the error in the code to correctly select rows where 'address' is NULL.

MySQL

SELECT * FROM customers WHERE address [1];

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

A!= NULL

BIS NOT NULL

C= NULL

DIS NULL

Attempts:

3 left

💡 Hint

Common Mistakes

Using '=' or '!=' operators with NULL

Confusing IS NULL and IS NOT NULL

Using = NULL causes an error or no results. Use IS NULL to check for NULL values.

4fill in blank

hard

Fill both blanks to select rows where 'birthdate' is NOT NULL and 'nickname' IS NULL.

MySQL

SELECT * FROM profiles WHERE birthdate [1] AND nickname [2];

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

AIS NOT NULL

BIS NULL

C= NULL

D!= NULL

Attempts:

3 left

💡 Hint

Common Mistakes

Using '= NULL' or '!= NULL' instead of IS NULL or IS NOT NULL

Mixing up IS NULL and IS NOT NULL

Use IS NOT NULL to check for values present and IS NULL to check for missing values.

5fill in blank

hard

Fill all three blanks to select rows where 'email' IS NOT NULL, 'phone' IS NULL, and 'status' IS NOT NULL.

MySQL

SELECT * FROM users WHERE email [1] AND phone [2] AND status [3];

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

AIS NULL

BIS NOT NULL

C!= NULL

D= NULL

Attempts:

3 left

💡 Hint

Common Mistakes

Using '= NULL' or '!= NULL' operators

Confusing which columns should be NULL or NOT NULL

Use IS NOT NULL for columns that must have values and IS NULL for columns that must be empty.

Practice

1. What does the SQL condition column_name IS NULL check for in a table?

easy

A. It checks if the column value is missing or empty.

B. It checks if the column value is zero.

C. It checks if the column value is a blank string.

D. It checks if the column value is greater than zero.

Solution

1. Step 1: Understand the meaning of NULL in SQL

   NULL means the value is missing or unknown, not zero or empty string.

2. Step 2: Analyze the condition IS NULL

   This condition returns true only when the column has no value (NULL).

3. Final Answer:

   It checks if the column value is missing or unknown. -> Option A

4. Quick Check:

   IS NULL means missing value check [OK]

Hint: IS NULL means value is missing, not zero or empty [OK]

Common Mistakes:

• Confusing NULL with zero or empty string
• Using = NULL instead of IS NULL
• Thinking NULL means zero

2. Which of the following is the correct syntax to select rows where the column email is NOT NULL?

easy

A. SELECT * FROM users WHERE email IS NOT NULL;

B. SELECT * FROM users WHERE email = NOT NULL;

C. SELECT * FROM users WHERE email != NULL;

D. SELECT * FROM users WHERE NOT email = NULL;

Solution

1. Step 1: Recall correct syntax for NOT NULL check

   In SQL, to check for NOT NULL, use IS NOT NULL.

2. Step 2: Evaluate each option

   SELECT * FROM users WHERE email IS NOT NULL; uses correct syntax. Others use invalid or incorrect operators with NULL.

3. Final Answer:

   SELECT * FROM users WHERE email IS NOT NULL; -> Option A

4. Quick Check:

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

Hint: Use IS NOT NULL, not = or != with NULL [OK]

Common Mistakes:

• Using = NULL or != NULL instead of IS NULL/IS NOT NULL
• Forgetting IS keyword
• Using NOT email = NULL which is invalid

3. Given the table orders with columns order_id and shipped_date, what will this query return?

SELECT order_id FROM orders WHERE shipped_date IS NULL;

medium

A. All orders that have a shipped date recorded.

B. All orders that have not been shipped yet.

C. All orders with shipped_date equal to zero.

D. Syntax error due to IS NULL usage.

Solution

1. Step 1: Understand what IS NULL filters

   The condition shipped_date IS NULL selects rows where shipped_date is missing.

2. Step 2: Interpret the meaning in context

   Missing shipped_date means the order has not been shipped yet.

3. Final Answer:

   All orders that have not been shipped yet. -> Option B

4. Quick Check:

   IS NULL finds missing shipped dates = unshipped orders [OK]

Hint: IS NULL finds missing data, here unshipped orders [OK]

Common Mistakes:

• Thinking IS NULL returns shipped orders
• Confusing NULL with zero or empty string
• Assuming syntax error with IS NULL

4. You wrote this query but it returns no rows:

SELECT * FROM employees WHERE manager_id = NULL;

What is the error and how to fix it?

medium

A. Add quotes around NULL like = 'NULL'.

B. Change manager_id to IS NOT NULL.

C. Use IS NULL instead of = NULL to check for NULL values.

D. Use != NULL to find rows with NULL.

Solution

1. Step 1: Identify incorrect NULL comparison

   Using = NULL always returns false because NULL is not equal to anything.

2. Step 2: Correct syntax to check NULL

   Use IS NULL to check for NULL values properly.

3. Final Answer:

   Use IS NULL instead of = NULL to check for NULL values. -> Option C

4. Quick Check:

   Use IS NULL, not = NULL, for NULL checks [OK]

Hint: Always use IS NULL, never = NULL for missing values [OK]

Common Mistakes:

• Using = NULL or != NULL instead of IS NULL/IS NOT NULL
• Adding quotes around NULL
• Confusing IS NULL with IS NOT NULL

5. You want to find customers who have provided a phone number but no email. Which query correctly uses IS NULL and IS NOT NULL to do this?

SELECT * FROM customers WHERE phone IS NOT NULL AND email IS NULL;

hard

A. This query has syntax errors with IS NULL usage.

B. This query returns customers missing both phone and email.

C. This query returns customers with both phone and email.

D. This query returns customers with phone and missing email.

Solution

1. Step 1: Analyze the conditions

   phone IS NOT NULL selects customers who provided a phone number.

2. Step 2: Check the email condition

   email IS NULL selects customers who did not provide an email.

3. Step 3: Combine conditions with AND

   Both conditions together find customers with phone but no email.

4. Final Answer:

   This query returns customers with phone and missing email. -> Option D

5. Quick Check:

   IS NOT NULL and IS NULL combined filter correctly [OK]

Hint: Combine IS NOT NULL and IS NULL with AND for mixed filters [OK]

Common Mistakes:

• Mixing AND with OR incorrectly
• Using = NULL instead of IS NULL
• Confusing which column to check for NULL