Understanding the Difference Between UNION and UNION ALL!

If you work with SQL, you've likely used UNION and UNION ALL to combine results from multiple queries. However, understanding their unique characteristics can lead to more efficient data handling and optimized queries. Here’s a quick breakdown:

UNION 💡

Combines the results of two or more SELECT statements and removes duplicate rows.

Useful when you want a unique set of records across all queries.

Since UNION performs a distinct operation, it can have a slight performance hit, especially on large datasets.

UNION ALL 🚀

Also combines results from multiple queries but retains all duplicates.This means it’s faster than UNION as it doesn’t require the additional operation of filtering out duplicates. Ideal when duplicates are acceptable or needed for analytics, and performance is a priority.

🔍 Examples:

SELECT employee_name FROM employees_in_us
UNION
SELECT employee_name FROM employees_in_uk;

SELECT employee_name FROM employees_in_us
UNION ALL
SELECT employee_name FROM employees_in_uk;

When to use which?

Use UNION when you need unique records.
Use UNION ALL for better performance when duplicates are okay!

Understanding these differences can make a huge impact on the efficiency and clarity of your SQL queries. Whether you're optimizing for performance or deduplication, choosing the right operator is key! 🔑