What are the different ways for Oracle to remove duplicates?

There are several methods to deduplicate data in Oracle databases.

1. By utilizing the DISTINCT keyword: Using the DISTINCT keyword in a SELECT statement can eliminate duplicate records.
2. Using the GROUP BY statement: By grouping identical records and applying aggregate functions (such as COUNT, SUM, AVG, etc.) to these groups of records, duplicate entries can be removed.
3. Use the ROW_NUMBER() function: assign a unique number to each record by using the ROW_NUMBER() function in the SELECT statement, then filter out the duplicate records by selecting only those with a row number of 1.
4. By using the UNION/UNION ALL operator: the UNION operator combines the results of multiple SELECT statements and removes duplicate records, while the UNION ALL operator does not remove duplicate records.
5. Utilize EXISTS subquery: Use an EXISTS subquery to determine if a record exists in another query result in order to eliminate duplicate records.

The above are several commonly used methods for deduplication in Oracle. Choose the appropriate method based on specific needs and data characteristics.