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 17

UNION SELECT

Combining results from multiple queries into a single result set.

IN THIS CHAPTER

UNION is used to combine results of two or more SELECT queries
Each SELECT must have the same number of columns
Column data types must be compatible
UNION removes duplicate rows by default
Use UNION ALL to include duplicates

🌟 Think of it this way: You have two lists — one of customers and one of employees. UNION merges them into a single list. It’s like stacking results on top of each other.

17.1 - UNION

Combine customer names and employee names into one list (duplicates removed).

SQL

SELECT name FROM customers
UNION
SELECT name FROM employees;

17.2 - UNION ALL

Combine results including duplicates.

SQL

SELECT name FROM customers
UNION ALL
SELECT name FROM employees;

✅ Pro Tip: Use UNION when you want unique results. Use UNION ALL when performance matters and duplicates are allowed.

Exercise 👇

Exercise:

Tasks

1.👉Combine customer and employee names (unique values)

2.Combine customer and employee names (include duplicates)

3.Combine cities and departments

4.Combine countries and departments

5.Combine filtered results from two tables

6.Sort combined results

7.Sort combined results including duplicates

8.Remove duplicates from same table using UNION

9.Remove duplicates from employees

10.Keep duplicates using UNION ALL

Stuck? Read this task's