Whether to build the data warehouse first or the data mart first?

That is one of the all time debated question in data warehousing. What Ralph Kimball said was "The data warehouse is nothing more than the union of all the data marts". However there are a lot of confusion in the fact that whether an organization has to start building a data warehouse or just start with a data mart and then expand. There are supporting facts for both approaches.

The Data Warehouse

A "data warehouse" will typically contain the full range of business intelligence available to a company from all sources. That data consists of transaction-processing records, corporate and marketing data, and other business operations information; for example, a bank might include loans, credit card statements, and demand deposits data, along with basic customer information. This internal data is frequently combined with statistical and demographic information obtained from outside sources. The cross-divisional nature of the data on file explains why a data warehouse is often called an "enterprise warehouse" -- because the wealth of data it gathers supports the informational needs of the corporate enterprise as a whole.

The Data Mart

Here we move to the next level down in the information hierarchy. A company's marketing, purchasing and finance departments will all make use of data stored in the enterprise warehouse. In many cases they will use the same data, but each department will massage that data in different ways. So each department sets up its own "data mart" designed to extract data from the enterprise warehouse. The key point here is that each mart processes the data in a form which suits its own departmental needs.

Data mart vs Data warehouse

• Data warehouse focuses on enterprise wide across all or many of the subject areas of the organization
• Data mart is restricted to single business process or single business group
• Union of data mart is nothing but a data warehouse
• The decision to set up a mart usually originates in that part of an organization with the most business "pain", and, thus, the opportunity for greatest gain. A warehouse usually comes into being when a senior executive notices a business problem recurring in several departments. Subsequent discussion reveals a greater need for cross-divisional data analysis than anyone thought. So a warehouse is born to collectively help all divisions behave as a single corporation.