Why WHERE with IS NULL and IS NOT NULL in SQL? - Purpose & Use Cases

The Big Idea

What if you could instantly find all missing pieces in your data without endless searching?

The Scenario

Imagine you have a big list of customer orders on paper. Some orders have missing delivery dates, but you want to find only those missing dates or only those that have dates filled in. Doing this by scanning each paper manually is tiring and slow.

The Problem

Manually checking each order for missing or present dates is error-prone and takes a lot of time. You might miss some or mix them up, especially if the list is very long.

The Solution

Using WHERE IS NULL and WHERE IS NOT NULL in SQL lets you quickly and accurately find rows with missing or present values. This saves time and avoids mistakes.

Before vs After

✗ Before

Check each row one by one for missing values.

✓ After

SELECT * FROM orders WHERE delivery_date IS NULL;
SELECT * FROM orders WHERE delivery_date IS NOT NULL;

What It Enables

This lets you instantly filter data based on whether information is missing or available, making data handling smarter and faster.

Real Life Example

A store manager wants to see all orders that haven't been shipped yet (missing shipping date) to follow up quickly.

Key Takeaways

Manually finding missing data is slow and error-prone.

IS NULL and IS NOT NULL help filter missing or present values easily.

This makes data queries faster and more reliable.

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