Data Warehousing:

Data Warehousing simply means data bank i.e. repository of all the related data. Data warehouse is a related data grouped together and organized so that it can easily be maintained, retrieved, analyzed, synthesized, edited, deleted and can easily be used for the various purposes for further understanding the data. It may be differentiate with other data that is grouped together to meet current business ethics such as order and payment transactions; also this data will also become part of a data warehouse. Few basic reasons why many organizations implement data warehouses are:

• To perform server/disk bound tasks associated with querying and reporting on servers/disks not used by transaction processing systems
• To use data models and/or server technologies that speed up querying and reporting and that are not appropriate for transaction processing
• To provide an environment where a relatively small amount of knowledge of the technical aspects of database technology is required to write and maintain queries and reports and/or to provide a means to speed up the writing and maintaining of queries and reports by technical personnel
• To provide a repository of "cleaned up" transaction processing systems data that can be reported against and that does not necessarily require fixing the transaction processing systems
• To make it easier, on a regular basis, to query and report data from multiple transaction processing systems and/or from external data sources and/or from data that must be stored for query/report purposes only
• To provide a repository of transaction processing system data that contains data from a longer span of time than can efficiently be held in a transaction processing system and/or to be able to generate reports "as was" as of a previous point in time
• To prevent persons who only need to query and report transaction processing system data from having any access whatsoever to transaction processing system databases and logic used to maintain those databases

1. Subject-oriented: Data in the database is organized in such a way that all the related data elements are from the same real-world event.
2. Time-variant: Data in the database is organized such that the changes to the data in the database are tracked and recorded so that reports can be produced showing changes with the time.
3. Non-volatile: Data in the database is organized such as the data is never over-written or deleted, ie. the data is static, read-only, but can be retained for future use.