Set Operators
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Set operators are used to combine the results of two (or more) SELECT statements. Valid set operators in Oracle 11g are UNION, UNION ALL, INTERSECT, and MINUS. When used with two SELECT statements, the UNION set operator returns the results of both queries. However, if there are any duplicates, they are removed, and the duplicated record is listed only once. To include duplicates in the results, use the UNION ALL set operator. INTERSECT lists only records that are returned by both queries; the MINUS set operator removes the second query’s results from the output if they are also found in the first query’s results. INTERSECT and MINUS set operations produce unduplicated results.