Sự khác biệt giữa UNION và UNION ALL trong SQL: Khi nào nên sử dụng?

In SQL, combining data from multiple tables is a common task. Two frequently used operators for this purpose are UNION and UNION ALL. While they both merge data into a single result set, understanding their differences is crucial for writing efficient and accurate queries. This discussion delves into the nuances of UNION and UNION ALL, exploring their functionalities, use cases, and performance implications.

Choosing between UNION and UNION ALL depends on the specific requirements of your SQL query. When duplicate rows are irrelevant and speed is paramount, UNION ALL is the preferred choice. Conversely, when a distinct result set is necessary, UNION should be used, keeping in mind the potential performance overhead. By understanding the characteristics of each operator, you can make informed decisions to optimize your SQL queries for both accuracy and efficiency.