Final answer:
Oracle derives column names for set operations from the first SELECT statement in the operation, taking the names or aliases defined there. The queries combined must have the same number and order of columns with compatible data types.
Step-by-step explanation:
When using set operations in Oracle, such as UNION, INTERSECT, and MINUS, the column names for the resulting dataset are derived from the first SELECT statement in the operation. Each set operation combines the results of two or more queries into a single result set. For the column names to be correctly derived, the number and order of the columns must be the same in all queries involved in the set operation, and their data types must be compatible. Oracle does not, however, perform a name check for matching column names across SELECT statements; it relies on the order and data types of the columns.
When the set operations are executed, if you do not explicitly assign column aliases in the first SELECT statement, Oracle will use the column names directly from the tables referenced in that query. If you provide aliases in the first SELECT statement, those aliases become the column names in the output. Therefore, it is often a best practice to provide clear and descriptive aliases to ensure that the result set columns are named appropriately, which can be helpful for users to understand the data.