To combine data from different records in your query you need to use the <link-entity> element. This is equivalent to a join in SQL.
As promised, I’m starting a series of blog posts covering various aspects of FetchXML and the humble <attribute> element seems like a good place to start.
MSDyn365 MVP Jonas Rapp threw down a challenge on Twitter last week: True, I’ve been ignoring for too long…If you know some C# and some #MSDyn365 WebAPI queries – please help…! 😊Pull Requests accepted! https://t.co/7iHp6XzQbf — Jonas Rapp ᴹᴠᴾ 🇸🇪 (@rappen) March 4, 2019