How can I learn ETL testing?
Testing is testing, doesn't matter what's the subject of your test.
Having said that, the main principle in ETL systems is DATA INTEGRITY.
So, if you want to make sure your system is working correctly:
First - understand how your data should look (your point of reference ) then begin to move as few components as possible in each iteration. In between each iteration be sure that your data is the same before and after it was transported. Rinse and repeat.
However, this can be long drawn out process and it really doesn’t have to be that way. There are several tools and automated methodologies that you can build out.