How does the UNION ALL operator differ from UNION in SQL?

The UNION ALL operator in SQL includes duplicate records, while the UNION operator does not.

In SQL, the UNION and UNION ALL operators are used to combine the result sets of two or more SELECT statements. However, they handle duplicate records differently. The UNION operator automatically removes duplicate rows from the results, effectively making every row in the result set unique. On the other hand, the UNION ALL operator does not remove duplicates, and instead, it includes all records from all the SELECT statements, even if they are identical.

The UNION operator works by comparing each record in the result set with every other record to identify and remove duplicates. This can be a time-consuming process, especially when dealing with large data sets. Therefore, if performance is a concern and you know that there will be no duplicate records, or if duplicate records are acceptable in your result set, it would be more efficient to use UNION ALL.

In contrast, the UNION ALL operator simply combines all records from all SELECT statements, without checking for duplicates. This makes it faster than UNION, but it also means that your result set could include duplicate records. If you need to ensure that every record in your result set is unique, you should use UNION instead.

In summary, whether you should use UNION or UNION ALL depends on your specific needs. If you need to eliminate duplicate records from your result set, use UNION. If you want to include all records, regardless of whether they are duplicates, and you want to do it as quickly as possible, use UNION ALL.