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Data warehouses are designed for quick access to large amounts of historical data. Read operations dominate over write operations. Under these conditions, normalization takes a back seat to performance optimization. A different design methodology, called dimensional design is used when planning a data warehouse.

There are two common categories of schemas used in data warehousing: star schemas and snow flake schemas. A star schema has a central fact table, surrounded by dimension tables. The fact table contains columns called measures, which are aggregated in queries. The fact table is related to the dimension tables. The dimension tables may have levels, which are implemented as columns. For example, a dimension table named Location may contain columns for Continent, Country, StateProvince and City. This dimension table is not normalized. If you normalize the dimension tables, then each level is placed in its own table. Normalizing the dimension tables results in a snow flake schema.

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