I had the same concerns ever since I started working with openEHR but I accepted the wisdom of experienced people that using pure RDBMS is not possible for openEHR CDRs. I also agree it is not trivial at all.
RDBMS and SQL are what people are used to. Hierarchical structure of openEHR is a great barrier to most of them (understandably).
I was always curious whether there is a way to flatten the hierarchy of openEHR data to fit into regular RDBMS tables. This is why I’m trying to do it (what can I say - I get bored sometimes 
).
I’m happy to share my findings with you or the data analyst but please be aware that according to other responses it might not work. However, learning about Better’s module “that allows to extract data to a relational database” gives me some hope.