underscore (_), pattern-matching character UNION ALL set operator 2nd comparing tables UNION clause, data sets, creating custom union compatibility conditions UNION operation compared to UNION ALL UNION query UNION set operator 2nd UNIQUE SINGLE REFERENCE cell reference Universal Coordinated Time (UTC) UNTIL option UPDATE semantics UPDATE statement CASE expression collections and DECODE function DML inline views join views multiple-column subqueries selective aggregation WHERE clause and updating cells XML documents UPSERT semantics user-defined constructors, object types USER_UPDATABLE_COLUMNS USING clause UTC (Universal Coordinated Time)