SQL UNION / UNION ALL

UNION and UNION ALL combine results from two or more SELECT queries.
Both must have the same number of columns with compatible data types.

🔹 Syntax

-- Removes duplicates
SELECT column1, column2 FROM table1
UNION
SELECT column1, column2 FROM table2;

-- Keeps duplicates
SELECT column1, column2 FROM table1
UNION ALL
SELECT column1, column2 FROM table2;

🔹 Example: Merge Customers from Two Regions

-- Unique customers only
SELECT customer_name FROM customers_us
UNION
SELECT customer_name FROM customers_uk;

-- Include duplicates
SELECT customer_name FROM customers_us
UNION ALL
SELECT customer_name FROM customers_uk;

🔹 Key Differences

Feature	`UNION`	`UNION ALL`
Duplicates	Removed	Included
Performance	Slower (due to deduplication)	Faster
Use When	You want distinct results	You need all results

🧠 Quick Recap

Key Point	Explanation
`UNION`	Combines results & removes duplicates
`UNION ALL`	Combines results with duplicates
Column match	Same number and types of columns required
Use cases	Merge similar datasets (e.g., logs, users)

🧩 Use UNION for clean merged data
🧩 Use UNION ALL for full raw results