DataBolt

All Lessons

Introduction to SQL

SQL Lesson 1: Your Sample Database

SQL Lesson 2: SELECT — Reading data

SQL Lesson 3: WHERE — Filtering rows

SQL Lesson 4: AND, OR, NOT

SQL Lesson 5: BETWEEN, IN, LIKE

SQL Lesson 6: NULL — The Mystery Value

SQL Lesson 7: ORDER BY

SQL Lesson 8: LIMIT & OFFSET

SQL Lesson 9: Aggregate Functions

SQL Lesson 10: GROUP BY

SQL Lesson 11: HAVING

SQL Lesson 12: INNER JOINs

SQL Lesson 13: LEFT JOINs

SQL Lesson 14: RIGHT JOINs

SQL Lesson 15: SELF JOINs

SQL Lesson 16: UNION JOINs

SQL Lesson 17: Joining Multiple Tables

SQL Lesson 18: Subqueries

SQL Lesson 19: CTEs (WITH)

SQL Lesson 20: CASE Statements

SQL Lesson 21: Window Functions

SQL Lesson 22: String Functions

SQL Lesson 23: Date & Time Functions

SQL Lesson 24: INSERT, UPDATE, DELETE

SQL Lesson 25: CREATE TABLE & DDL

SQL Lesson 26: Indexes & Performance

SQL Lesson 27: Transactions & ACID

SQL Lesson 28: SQL Execution Order

SQL Lesson 29: 50 Practice Problems

CHAPTER 7

NULL - The Mystery Value

The concept that trips up almost every SQL beginner.

IN THIS CHAPTER

What NULL actually means (and what it does not mean)
IS NULL and IS NOT NULL — the only correct way to check for NULL
Why NULL = NULL is not TRUE in SQL
COALESCE to handle NULL gracefully

🌟 Think of it this way: NULL is the answer to 'what is the name of your 10th child?' — it is not 'nothing' or 'empty'. It is 'this value does not exist / is unknown.' SQL treats it completely differently from zero or empty string.

7.1 - Finding NULLs

Top-level employees with no manager

SELECT name, manager_id 
FROM employees
WHERE manager_id IS NULL;

7.2 - The NULL = NULL Trap

❌ Common Mistake: WHERE manager_id = NULL will ALWAYS return 0 rows. NULL = NULL evaluates to NULL (unknown), not TRUE. This is defined by the SQL standard. Always use IS NULL or IS NOT NULL.

SELECT * FROM employees WHERE manager_id = NULL; ----> Returns 0
rows - WRONG
SELECT * FROM employees WHERE manager_id IS NULL; ----> Returns 2
rows - CORRECT

7.3 - COALESCE

Show 'No Manager' where manager_id is NULL

SELECT name, COALESCE(CAST(manager_id AS CHAR), 'No Manager') AS 
       reports_to
FROM employees;

💡 Engineering Insight: COALESCE is invaluable in analytics. When calculating revenue per customer, a NULL (no orders) should show ₹0, not distort averages. COALESCE(SUM(total), 0) handles this cleanly.

Exercise 👇

Exercise:

Tasks

1.👉Find employees who do not have a manager (manager_id is NULL).

2.Find employees who have a manager (manager_id is NOT NULL).

3.Find employees who do not have a manager and are in Sales department.

Stuck? Read this task's