A typical use case in a Data Warehouse is that flat files are loaded into the Staging Area via external tables. The ETL process reads the contents of the file with a SELECT statement on an external table and writes it to the corresponding stage table. As long as the ETL tool connects directly to the schema where the external table is created, this is simple. But how about different schemas?
…does not exist. Each Data Warehouse is different. Not only the known (or unknown) analytical requirements and the number, complexity and structure of the source systems have an impact on the architecture and design of a Data Warehouse.