I have requirement where i need to display different data from 2 classes in one single RD but in different rows (like on pictures 1 and 2) .To achieve this using joins did not work, how can this be achieved? Does Platform have a functionality similar to SQL "Union" (analogous to a similar request like on picture 3)?
I don't know of any way to explicitly specify a union through the report definition rule interface.
My suggestion would be to create a database view that is populated using your union query, and base the report on that. Database views can be treated the same as database tables in Pega - i.e., you can map a class to a view as if it was a table. You can then create your report from that class.