In reply to Tatiana Pshenichnikov.

Thank you, Tatiana, for your comment. Of course, it can be done in one step as you suggested.
However, in that single step you read both data sets in their entirety. PROC APPEND, on the other hand, does not read the whole dataset and therefore runs much faster. You will see noticeable time savings when your sasdl.study2017 dataset is large.