Data Analysis With PostgreSQL: Analytic Aggregate Functions

SQL is the declarative language for asking questions of relational databases: you state which rows and columns you want and how tables relate, and the database engine works out how to retrieve them.

ETL is the pattern for moving data from the systems that produce it into the one place it is analysed: extract the records from each source, transform them into a common, cleaned shape, and load them into a warehouse; ELT keeps the same three steps and runs the transformation inside the warehouse after loading.

A data warehouse is a database built for analysis rather than for running the business: it holds copies of records from many operational systems, integrated under one set of definitions, kept as history rather than overwritten, and arranged so that questions across years and departments run fast.