A database warehouse is a collection of databases with information extracted from other databases. In some cases, end users cannot access corporate databases for security or legal reasons. In other cases, related information may be spread across multiple platforms such as mainframes and PC servers.

A database warehouse lets a database manager extract information from such databases and present it to users as a database whose form may or may not resemble the original database. A database warehouse can even provide summarized information.

For example, a corporate sales database contains a list of ordered products. The customer database contains shipping address information. A product description database includes product size and weight information. The product planning group wants the best shipping solution but needs to know the current requirements. The database manager can use a database warehouse product to extract the appropriate information from the respective databases, combining the information into a customized database for the product planning group. This database may contain only the necessary information, such as the city and state, plus a summary of units shipped to that area and their average size and weight. Product planning does not need to know what databases you accessed to retrieve the information.

Another simple example uses information from one reduced database that prevents users from accessing the private information. For example, an employee database could be the basis for an employee telephone directory. Anyone could access this part of the database, but the employee database with private information, such as salary and benefits, would remain secure.

A database warehouse typically includes a meta-directory, or information catalog. A meta-directory describes what data the database warehouse maintains, including the data type (integer, string, etc.) or column in a database. The meta-directory lets database warehouse users determine what databases to look at, based on their content.

The meta-directory presentation often lets users select both the source of information and an analysis and presentation method. Users often generate reports repeatedly, supplying new data or different selections. Alternatively, you can create custom applications to use the meta-directory to locate information, and the same application can then access the selected database.

A meta-directory typically consists of a database that the database warehouse extraction support maintains. You can adjust the presentation of the meta-directory to display information in terms that users can appreciate, including annotations such as the original location of extracted database information. Some database warehouse products or database servers use the Web server to present this information.

Database warehouses operate on a simple principle: Take information from one set of databases and put it in another. This process is an extraction. As always, the devil is in the details. The destination databases do not exist in realtime. You must define and schedule each extraction, and you can extract information frequently (e.g., hourly) or infrequently (e.g., once a month).

Database managers often organize database information for a specific application or set of applications. This practice may make the database unsuitable for human consumption or analysis. Database warehouse software lets a database manager become an information manager while letting users use the warehouse information directly.