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Query, Extract, Transform, Load

QETL (pronounced "kettle") refers to a set of processes for working with data. The term broadly covers four areas:

  • Query: Fetching and analyzing data on-demand, without removing it from the source.
  • Extract: Extracting data from a source system.
  • Transform: Data cleansing and manipulation to improve data quality and establish consistency.
  • Load: Moving data into a target system.

The processes are often combined in various patterns: ELT, ETL, and QETL.

ELT: Extract → Transform → Load​

ETL is a three-phase process where data is extracted, transformed (cleaned, sanitized, scrubbed) and loaded into an destination.

ELT: Extract → Load → Transform​

Extract, load, transform (ELT) is an alternative to ETL, often used with data lake implementations. In contrast to ETL, in ELT models the data is not transformed on entry to the data lake, but stored in its original raw format.

QETL: Query, Extract, Load, Transform​

The term QETL refers to the set of practices (which encompasses ETL and ELT), and also an approach. The QETL approach focuses on incremental loading, fetching and storing data on-demand, and dropping data when free space is needed. Transformations can be performed either on the source or the destination, and so the process can either be Q → T → E → L, or Q → E → L → T.