What is the purpose of union in Oracle?

In Oracle, UNION is an operator used to combine the result sets of two or more SELECT statements. Its purpose is to merge the results of two queries into one result set and eliminate duplicate rows.

Specifically, the UNION operator merges the result sets of two or more queries in column order, creating a new result set that includes all query results. If there are duplicate rows in the query results, the UNION operator automatically removes the duplicates, keeping only unique rows.

The UNION operator has the following characteristics:

1. The number of columns and data types in the result set must be consistent, otherwise an error will occur.
2. The UNION operator combines result sets based on column order, not the order of the queries.
3. The UNION operator by default eliminates duplicate rows, if you want to keep duplicate rows, you can use the UNION ALL operator.

The UNION operator is commonly used to merge the results of multiple queries into one result set, widely applied in data reporting, data analysis, and other scenarios.