In SQL, both UNION and UNION ALL are used to combine the results of two or more SELECT queries, but they differ in how they handle duplicates. The key difference is that UNION removes duplicate rows from the combined result set, while UNION ALL includes all rows from the queries, including duplicates. This distinction can significantly affect the performance and the result set, depending on the context in which you are using these operations.
When using UNION, the database system processes the result sets by first combining them and then eliminating any duplicate rows. This involves additional overhead, as the system needs to sort and compare the values to filter out the duplicates. For example, if you have two tables—Table A with values (1, 2, 3) and Table B with values (2, 3, 4)—running a UNION on both tables would result in the combined set (1, 2, 3, 4), with duplicates removed. This can be particularly useful when you only want unique entries from multiple sources.
Conversely, UNION ALL simply appends the results of the queries without any duplicate removal. Using the same example, executing UNION ALL on Table A and Table B would yield (1, 2, 3, 2, 3, 4), preserving all occurrences. This can improve performance because the database does not need to perform the extra work involved in filtering duplicates. It is especially useful when you expect or need duplicates in your results or when you’re working with large datasets where the added performance from skipping duplicate checks is beneficial. Therefore, choosing between UNION and UNION ALL should be based on your specific requirements regarding duplicates and performance considerations.