Learning about SAP ABAP CDS views

First thing, whether it is CDS or ABAP or anything pertaining to SAP, you can always find free courses on  https://open.sap.com/  referred as Open SAP. Besides this, there are lot of free help as well just Google it and the result list is huge.

There are various self-learning tutorials site that provides free study material with use cases that can also be referred - https://www.tutorialspoint.com/sap_abap/