Hos does Appian deal with accessing hierarchical tables(Entities)? Through Webse

You can create representations of relational data in Appian: forum.appian.com/.../Create_Custom_Data_Types and use Query Rules and Write to Data Store nodes to manipulate the DB data directly. Web Services are also an option.
I would highly recommend you attend some Appian training (www.appian.com/.../bpm-training-overview.jsp) to learn some basics of Appian application design.