ETL processes are the transfer of data from source systems into a target system:
- Extract: The reading of data from the source systems.
- Transform: Various data modifications, customization of data types, elimination of duplicates, aggregation, and standardization of heterogeneous data structures.
- Load: Writing data in the target system.
The source systems usually involve one or more operational databases and their data is combined into a target system. It begins with an initial load and the periodic loading of data occurs incrementally to keep the data warehouse up-to-date.
For more information about how ETL processes are used to load data, see Loading Data.