UNION vs UNION ALL: The Key SQL Difference for Merging Datasets UNION vs UNION ALL: The Key SQL Difference for Merging Datasets A simple keyword that can dramatically affect your query's performance. Welcome! When you need to combine the results of two or more `SELECT` statements into a single result set, you'll use a `UNION` clause. But if you've ever had a query that runs slowly, the culprit might be a single, seemingly minor word. Understanding the crucial difference between **UNION** and **UNION ALL** is essential for writing efficient and correct SQL queries. The UNION Operator The **`UNION`** operator is used to combine the result sets of two or more `SELECT` statements. The key characteristic of `UNION` is that it automatically **removes duplicate rows** from the final result set. It's like a combination of a `SELECT` and a `DISTINCT` ...