John, if you have several languages defined on your site - you will get several documents, 1 document per language. For 1 node in the tree you can have a few documents, one per language.
You may have several sites hosted in the same DB
Select * from View_CMS_Tree_Joined where NodeSiteID = 1 - means give me all the documents that belong to the site with id =1,
P.S. Ah yes, Page Id in the admin interface and documentId in the DB is the same thing :)
Select * from View_CMS_Tree_Joined where DocumentId =81
Document ID (Page ID) - is unique, so if use this Page id(DocumentID) from the proeprties screen - you will get the exact document:
Select DocumentPageDescription , DocumentPageTitle, DocumentUrlPath
from View_CMS_Tree_Joined where DocumentId =81