What is the methodology and process followed for ETL testing in Data warehouse environment?

Like ETL SPEC they create a document containing the source table (Schema name) and a target table (another schema name) with the logic used in transforming source to target. We have to write database query with the logic contained in the document using source schema and take its output. Now write a simple select statement in the target schema and take its output. Compare the two outputs if they are same well and fine else it's a bug.

Database knowledge is a must for ETL testing.