Hi Tobias,
Subquries do exist. Although in quite a restrictive way. The projection list of a subquery cannot be used outside the subquery and not inside the join condition. So it can be used only in the condition clauses of a query.
Check out the documentation here. The same applies for CDS views too.
https://help.sap.com/abapdocu_750/en/abenwhere_logexp_subquery.htm
Regards,
Harish R