Data warehouse is three tiers architecture. Data Warehouse also called as Enterprise data warehouse (EDW). It is a system used for data analysis and reporting and business intelligence. Data warehouses are Central Repositories of integrated data from one or more disparate sources. They also store historic and current data in one single place. The data warehousing concept was intended to provide an architectural model for the flow of data from operational systems to decision support environments.

ETL and ELT are the two main Approached used to build a data warehouse system. Extract, Transform, Load (ETL) data warehouse uses for data integration, staging and access layers for its key functions. Extract Load, Transform (ELT) data warehouse is a separate ETL tool for data transformation. Instead, it maintains a staging area inside the data warehouse itself.

• Top Down method
• Bottom Up method
• Hybrid method

• Used to help an organization make decisions better and faster
• improving performance of both systems
• Consolidates data from many of the sources
• Quality of Data, consistency and accuracy
• Historical Intelligence
• Separates analytics processing from transactional databases