WHERE with IS NULL and IS NOT NULL in SQL - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does the SQL condition IS NULL check for?

It checks if a column's value is missing, meaning it has no data stored (NULL).

beginner

How do you select rows where a column is NOT empty in SQL?

Use WHERE column_name IS NOT NULL to find rows where the column has some value.

intermediate

Why can't you use = NULL to check for NULL values in SQL?

Because NULL means unknown or missing data, and = compares values. Use IS NULL instead.

beginner

Write a SQL query to find all employees without a manager (manager_id is NULL).

SELECT * FROM employees WHERE manager_id IS NULL;

beginner

What is the difference between IS NULL and IS NOT NULL in a WHERE clause?

IS NULL finds rows where the column has no value. IS NOT NULL finds rows where the column has some value.

Which SQL clause correctly finds rows where the 'email' column has no value?

A WHERE email IS NOT NULL

B WHERE email = NULL

C WHERE email = ''

D WHERE email IS NULL

IS NULL checks for missing values. = NULL is invalid for this check.

What does WHERE phone_number IS NOT NULL do?

A Finds rows where phone_number is missing

B Finds rows where phone_number has a value

C Finds rows where phone_number equals zero

D Finds rows where phone_number is empty string

IS NOT NULL filters rows with any value present in the column.

Why is WHERE column = NULL not correct in SQL?

A Because NULL means unknown and can't be compared with =

B Because NULL is a reserved keyword

C Because = only works with numbers

D Because NULL is a string

NULL means unknown or missing, so = comparison doesn't work; use IS NULL.

Which query finds all records where 'birthdate' is known (not missing)?

A SELECT * FROM table WHERE birthdate IS NOT NULL

B SELECT * FROM table WHERE birthdate = NULL

C SELECT * FROM table WHERE birthdate IS NULL

D SELECT * FROM table WHERE birthdate = ''

IS NOT NULL filters rows where the column has a value.

If you want to find rows where a column 'score' is missing, which is correct?

A WHERE score = ''

B WHERE score = 0

C WHERE score IS NULL

D WHERE score IS NOT NULL

IS NULL finds missing or unknown values.

Explain how to use IS NULL and IS NOT NULL in a SQL WHERE clause and why they are important.

Think about how databases store missing information.

Write a simple SQL query to find all records where the 'address' field is missing.

Use IS NULL to check for missing data.

Practice

1. Which SQL clause correctly finds rows where the email column has no value stored (is missing)?

easy

A. WHERE email = NULL

B. WHERE email = ''

C. WHERE email IS NULL

D. WHERE email IS NOT NULL

Solution

1. Step 1: Understand NULL meaning in SQL

   NULL means no value or missing data, not an empty string or zero.

2. Step 2: Use correct syntax to check NULL

   To check for NULL, use IS NULL, not = NULL or empty string.

3. Final Answer:

   WHERE email IS NULL -> Option C

4. Quick Check:

   Check NULL with IS NULL = WHERE email IS NULL [OK]

Hint: Use IS NULL to find missing values, never = NULL [OK]

Common Mistakes:

• Using = NULL instead of IS NULL
• Confusing empty string '' with NULL
• Using IS NOT NULL to find missing values

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

easy

A. SELECT * FROM contacts WHERE phone IS NOT NULL;

B. SELECT * FROM contacts WHERE phone = NOT NULL;

C. SELECT * FROM contacts WHERE phone != NULL;

D. SELECT * FROM contacts WHERE phone NOT NULL;

Solution

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

   SQL uses IS NOT NULL to find rows where value exists.

2. Step 2: Identify syntax errors in other options

   Options B, C, D use invalid syntax like = NOT NULL or != NULL or missing IS.

3. Final Answer:

   SELECT * FROM contacts WHERE phone IS NOT NULL; -> Option A

4. Quick Check:

   Use IS NOT NULL to find existing values = SELECT * FROM contacts WHERE phone IS NOT NULL; [OK]

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

Common Mistakes:

• Using = NOT NULL instead of IS NOT NULL
• Using != NULL which is invalid
• Omitting IS keyword

3. Given the table employees with columns id, name, and manager_id where some employees have no manager (NULL), what will this query return?

SELECT name FROM employees WHERE manager_id IS NULL;

medium

A. All employees who have a manager assigned

B. All employees who do not have a manager assigned

C. All employees including those with and without managers

D. Syntax error due to IS NULL usage

Solution

1. Step 1: Understand IS NULL condition

   The condition manager_id IS NULL filters rows where manager_id has no value (missing).

2. Step 2: Interpret query result

   The query returns names of employees without any manager assigned (NULL manager_id).

3. Final Answer:

   All employees who do not have a manager assigned -> Option B

4. Quick Check:

   IS NULL filters missing values = All employees who do not have a manager assigned [OK]

Hint: IS NULL finds missing values, so returns employees without managers [OK]

Common Mistakes:

• Thinking IS NULL finds rows with values
• Confusing NULL with empty string
• Assuming syntax error with IS NULL

4. Identify the error in this query that tries to select rows where address is NOT NULL:

SELECT * FROM customers WHERE address != NULL;

medium

A. Should use = NULL instead of != NULL

B. Missing quotes around NULL

C. No error; query is correct

D. Using != NULL is invalid; should use IS NOT NULL

Solution

1. Step 1: Check NULL comparison syntax

   In SQL, NULL cannot be compared with = or != operators.

2. Step 2: Use correct syntax for NOT NULL

   The correct way is IS NOT NULL to check for non-missing values.

3. Final Answer:

   Using != NULL is invalid; should use IS NOT NULL -> Option D

4. Quick Check:

   Use IS NOT NULL, not != NULL [OK]

Hint: Never use = or != with NULL; use IS (NOT) NULL [OK]

Common Mistakes:

• Using != NULL or = NULL
• Adding quotes around NULL
• Assuming != NULL works like other values

5. You have a table orders with columns order_id, customer_id, and shipped_date. Some orders are not shipped yet and have shipped_date as NULL. Which query lists all orders that have been shipped?

hard

A. SELECT * FROM orders WHERE shipped_date IS NOT NULL;

B. SELECT * FROM orders WHERE shipped_date IS NULL;

C. SELECT * FROM orders WHERE shipped_date != NULL;

D. SELECT * FROM orders WHERE shipped_date = NULL;

Solution

1. Step 1: Understand meaning of shipped_date NULL

   NULL means order not shipped yet, so shipped orders have non-NULL dates.

2. Step 2: Use correct syntax to find shipped orders

   Use IS NOT NULL to find rows where shipped_date has a value.

3. Final Answer:

   SELECT * FROM orders WHERE shipped_date IS NOT NULL; -> Option A

4. Quick Check:

   IS NOT NULL finds shipped orders = SELECT * FROM orders WHERE shipped_date IS NOT NULL; [OK]

Hint: IS NOT NULL finds rows with values, perfect for shipped orders [OK]

Common Mistakes:

• Using = NULL or != NULL which are invalid
• Using IS NULL to find shipped orders
• Confusing NULL with empty string