Data entering the data warehouse comes from operational environment in almost every case.Data warehousing provides architectures and tools for business executives to syste-matically organize understand ,and use their data to make stragetic decisions.A large number of organizations have found that data warehouse systems are valuable tools in today’s competive,fast-evolving world. In the last several years ,many firms have spent millions of dollars in building enterprise wide data warehouses. Many people feel that with competition mounting in every industry ,data warehousing is the latest must have marketing weapon –a way to keep customers by learning more about their needs.

Data warehousing is a more formalised methodology of these techniques. For example, many sales analysis systems and executive information systems (EIS) get their data from summary files rather then operational transaction files. The method of using summary files instead of operational data is in essence what data warehousing is all about. Some data warehousing tools neglect the importance of modelling and building a datawarehouse and focus on the storage and retrieval of data only.