What is Data Warehouse

• Before starting learning Data Warehouse, we should be aware of the Father of Data Warehouse, So Ralph Kimball and Bill Inmon are known as Father of Data Warehouse.
• Ralph Kimball, is a proponent of an approach frequently considered as bottom-up ,to data warehouse design. In the so-called bottom-up approach data marts are first created to provide reporting and analytical capabilities for specific business processes. Data marts contain atomic data and, if necessary, summarized data. These data marts can eventually be unioned together to create a comprehensive data warehouse. The combination of data marts is managed through the implementation of what Kimball calls "a data warehouse bus architecture.
• Business value can be returned as quickly as the first data marts can be created. Maintaining tight management over the data warehouse bus architecture is fundamental to maintaining the integrity of the data warehouse.

• Bill Inmon, one of the first authors on the subject of data warehousing, has defined a data warehouse as a centralized repository for the entire enterprise. Inmon is one of the leading proponents of the top-down approach to data warehouse design, in which the data warehouse is designed using a normalized enterprise data model. "Atomic" data, that is, data at the lowest level of detail, are stored in the data warehouse. In the Inmon vision the data warehouse is at the center of the "Corporate Information Factory" (CIF), which provides a logical framework for delivering business intelligence (BI) and business management capabilities. The CIF is driven by data provided from business operations.

            Inmon states that the data warehouse is:

• Subject-oriented 

–        The data in the data warehouse is organized so that all the data elements relating to the same real-world event or object are linked together.

• Time-variant 

–        The changes to the data in the data warehouse are tracked and recorded so that reports can be produced showing changes over time.

• Non-volatile 

            - Data in the data warehouse is never over-written or deleted - once committed, the data is static, read-only, and retained for future reporting.

• Integrated 

            - The data warehouse contains data from most or all of an organization's operational systems and this data is made consistent.