You have two ANSI-SQL tables with identical columns and need to combine them into one result set, removing duplicate rows. Which SQL operation should you use?

The UNION operator combines the result sets of two or more SELECT statements into a single result set, and importantly, it automatically removes duplicate rows. This directly addresses the requirement of combining tables and removing duplicates. JOIN is used to combine rows from two or more tables based on a related column between them, not to stack entire result sets. Nested WITH statements (Common Table Expressions) are used for readability and breaking down complex queries, not for combining result sets in this manner. UNION ALL combines result sets like UNION, but it includes all duplicate rows, which contradicts the requirement to remove duplicates.