In database management, extract, transform, load (ETL) is a programming tool that consists of three separate functions. ETL is often used to acquire a temporary subset of data for reports or other purposes. First, the extract function reads data from a specified source database and removes a desired subset of data. Next, the transform function works with the acquired data - using rules or lookup tables, or creating combinations with other data - and converts it to the desired state. Finally, the load function writes the resulting data (either all of the subset or just the changes) to a target database, which may or may not previously exist. In addition to acquiring a data subsets for reports, ETL is also used to migrate data from one database to another and populating data warehouses. ETL can be contrasted with ELT (Extract, Load, Transform) which transfers raw data from a source server to a data warehouse on a target server and then prepares the information for downstream uses.
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