What ETL is not?
ETL should not be confused with a data creation process. It never creates new data. If a list of hundred employees is being loaded, one more employee cannot be added to the list and make it hundred and one. Or if last name of customer is absent an arbitrary last name cannot be substituted.
Data warehouses are not OLTP systems. Duplication of calculations in Source system & the data warehouse should not be attempted, as the numbers will be very difficult to match during QA. Also in future the process in the source system can change that will result in asynchronous data.
ETL cannot change the meaning of data. For example for sex ‘M’ and ‘F’ in source system sex flag to ‘1’ and ‘2’ is used in the Data Warehouse respectively. This is OK because this does not change the business meaning of the data. It only has changed the representation of the data.
