Most data in your database is not loaded directly in the database, but has been assembled from underlying raw data using queries. This is the T step in ELT to refine and combine data from different sources, to combine it into useful information. One table may be generated by an SQL query on the raw data, and that table may again be used by an ETL function to generate another dataset. This creates directional relationships between the various datasets, similar to a family tree with descendants and ancestors.