What is the purpose of the `WHEN NOT MATCHED BY SOURCE` clause in the Databricks merge operation?
The `WHEN NOT MATCHED BY SOURCE` clause in the Databricks merge operation is used to specify an action for the rows in the target table that do not have a matching row in the source table. This clause can be used to either update or delete these unmatched rows in the target table. It is recommended to add an optional conditional clause to avoid fully rewriting the target table.