What is a data warehouse? How does it differ from a database?

A data warehouse is a system used for reporting and data analysis. They are central repositories of integrated data from one or more disparate sources. They store current and historical data and are used for creating trending reports for senior management reporting such as annual and quarterly comparisons. Data in the warehouse is uploaded from the operational systems such as sales, marketing and others, and the upload is typically done at set intervals.

A database is an organized collection of data stored and accessed electronically. Databases are designed to offer an efficient way to store, retrieve and manage information. They are used to hold records or data, which are usually organized in tables. While they may contain aggregated data for reports, that's not their primary function.

A key difference between a data warehouse and a database is that data warehouses are primarily used for data analysis and therefore contain historical and aggregated data, while databases are used for transactional operations – to create, read, update, and delete operation. Data warehouses are optimized for read access, thus providing a fast way to analyze large amounts of data, while databases are designed to quickly record and retrieve specific data items. Another difference is the way they are modeled; data warehouses often use denormalized or partially denormalized schemas (such star schema) to optimize read performance, while transactional databases usually follow a fully normalized model for the sake of avoiding data redundancy and ensuring data consistency.