A data warehouse is the concept of data extracted from operational systems and made available as historical snapshots for ad-hoc queries and scheduled reporting. Characteristics that distinguish data in the data warehouse from data found in the operational environment are that it is: organized in such as way that relevant data is clustered together for easy access, several copies of the data from various points in time are kept together, and once the data is placed into the data warehouse it is not updated. Rather, the historical snapshots stored in the Data Warehouse are periodically refreshed with data from the operational databases.
Data Warehousing solutions must not only deliver answers quickly and accurately, they must also deliver answers to multiple users with differing interests. For example, an accounting department may be interested in the Return On Investment (ROI) of the latest direct-mail promotion, while the marketing department is more interested in analyzing the market penetration the promotion achieved.
