A data warehouse schema describes the organization of data within the [[data warehouse]]. Multiple schemas have been developed for various applications. Warehouse schemas typically include two elements - **fact tables**: stores quantitative data (measurable events or metrics) and the foreign keys that reference associated dimension tables. Typically very large stores of quantitative data - **dimension tables**: provides descriptive attributes related to dimensions in the fact table. They give context to the measures. Typically small stores of qualitative and categorical data. For example, a list of transactions may be stored in a fact table with foreign key references to a customer and products, whose details are stored in dimension tables. An individual unit in a data warehouse is called a [[data cube]]. ## star schema One fact table, multiple dimension tables. ## snowflake schema An expansion on the star schema with one fact table and a hierarchy of dimension tables. ## fact constellation schema Multiple fact tables, multiple shared dimension tables.