SolveWithSQL

Difficulty: Beginner
Estimated time: 10–15 minutes
SQL concepts: NULL, IS NULL, IS NOT NULL, filtering missing data
Goal: Learn how to correctly filter rows based on missing or present values.

The Scenario

After cleaning up missing values with COALESCE, a data analyst asks:

“Can we find only the rows that still have missing values?”

In SQL, NULL behaves differently from regular values.
To filter it correctly, you must use IS NULL or IS NOT NULL.

Database Table

products

column_name	type	description
product_id	INTEGER	Unique product ID
name	TEXT	Product name
category	TEXT	Product category
supplier_notes	TEXT	Optional notes from supplier (can be NULL)

Sample Data

product_id	name	category	supplier_notes
101	Wireless Mouse	Accessories	NULL
102	Mechanical Keyboard	Accessories	Best seller
103	27-inch Monitor	Displays	NULL
104	USB-C Hub	Accessories	Limited stock
105	Laptop Stand	Workspace	NULL

Your Task

Write a SQL query that returns:

• Product name
• supplier_notes

…but only for products where supplier_notes is missing.

Expected Output

name	supplier_notes
Wireless Mouse	NULL
27-inch Monitor	NULL
Laptop Stand	NULL

Constraints

• Use IS NULL to filter missing values
• Do not use = NULL
• Do not use SELECT *

Hint (Optional)

In SQL:

• = NULL ❌ does not work
• IS NULL ✅ is the correct way

The same applies to IS NOT NULL.

Explanation

• IS NULL checks whether a column has no value.
• Regular comparison operators (=, !=) do not work with NULL.
• This pattern is commonly used for:
  • Data quality checks
  • Finding incomplete records
  • Debugging pipelines

Common Mistakes

1. Using = NULL WHERE supplier_notes = NULL -- ❌ always false
2. Forgetting NULLs behave differently
3. Filtering after replacing NULLs with COALESCE
   • You must filter before replacing, or use the raw column.

Try:

1. Return products that do have supplier notes (IS NOT NULL)
2. Count how many products are missing notes
3. Combine IS NULL with LEFT JOIN to find missing relationships

Next Challenge

Beginner Challenge #14: Filtering Ranges with BETWEEN

🔗 View reference solution on GitHub
(After you’ve tried the challenge)

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