ETL vs ELT vs ETLT Flow Comparison

Extract, transform, and load (ETL) is a pipeline that cleans and reshapes data before it lands in the warehouse. Data flows from source systems such as relational databases into a middle processing layer, then into BigQuery as the warehouse sink. The order is fixed: first extract data from sources, then transform it by cleaning values, applying business rules, checking integrity, and creating aggregates, then load the finished result. Because heavy work runs outside the warehouse, warehouse compute stays free for concurrent queries. The tradeoff is extra tooling and failure points, since teams must use languages beyond SQL and each new source adds another place that can fail.