Data Warehouse is a repository of integrated information, available for queries and analysis. Data and information are extracted from heterogeneous sources as they are generated....This makes it much easier and more efficient to run queries over data that originally came from different sources.

Typical relational databases are designed for on-line transactional processing (OLTP) and do not meet the requirements for effective on-line analytical processing (OLAP). As a result, data warehouses are designed differently than traditional relational databases.

A Data Warehousing is defined in 2 ways by 2 authors named "Ralph Kimball" and "W.H.Inman"

According to Ralph Kimball, . A D.W.H is a relational database which is specially design for business analysis but not for running the business.

. An enterprise D.W.H is design to make decision process. Hence it is called Decision Support System.

. A Data Warehouse is design to only read operations required for business analysis but not for transactional process. Hence it is called Read Only Database.

According to W.H.Inman, A Data Warehouse is a,

1) Time variant Database

2) Non-Volatile Database

3) Integrated Database

4) Subject oriented Database

and a Data Warehouse is a historical database