Why UNION and UNION ALL in MySQL? - Purpose & Use Cases

The Big Idea

What if you could combine multiple lists perfectly with just one simple command?

The Scenario

Imagine you have two separate lists of customer names from two different stores, and you want to see all customers who visited either store. You try to write down all names by hand or copy-paste them into a single list.

The Problem

This manual method is slow and messy. You might accidentally write the same name twice or miss some names. It's hard to keep track and update the list when new customers come in.

The Solution

Using UNION and UNION ALL in SQL lets you combine these lists automatically. UNION removes duplicates, showing each customer once. UNION ALL keeps all entries, including duplicates, so you see every visit.

Before vs After

✗ Before

List1 = ['Alice', 'Bob']
List2 = ['Bob', 'Charlie']
Combined = []
for name in List1:
    if name not in Combined:
        Combined.append(name)
for name in List2:
    if name not in Combined:
        Combined.append(name)

✓ After

SELECT name FROM store1_customers
UNION
SELECT name FROM store2_customers

What It Enables

It makes combining data from multiple sources easy, fast, and accurate without manual errors.

Real Life Example

A company wants to see all unique email subscribers from two different newsletter lists to send a single campaign without duplicates.

Key Takeaways

Manual combining is slow and error-prone.

UNION merges lists and removes duplicates automatically.

UNION ALL merges lists and keeps all entries, including duplicates.