What is a Data Warehouse?

The primary concept behind data warehousing is that the data stored for business analysis can most effectively be accessed by separating it from the data in the operational systems. A data warehouse, therefore, is a collection of data gathered from one or more data repositories to create a new, central database. For example a hospital may create a data warehouse by extracting the operational data it has accumulated concerning patient information, lab results, drug use, length of stay, disease state, etc,. Data Warehousing is not just the data in the warehouse, but also the architecture and tools to collect, query, analyze and present information.
The characteristics of a data warehouse were first defined by W.H. Inmon who stated, “a data warehouse is subject-oriented, integrated, time-variant and non-volatile [data] collection in support of management decision making processes”. Let’s break that definition down:

• Subject-oriented: all relevant data concerning a subject is gathered and stored in a single database.
• Integrated: all data in the warehouse must be compatible with each other regardless of type or location.
• Time-variant: all data contains a reference to time so that the age of each piece of data can be determined.
• Non-volatile: the data does not change once it has been collected.