I am trying to put together an ER Diagram to design a fairly simple asset tag tracking/inventory management system for our servers/switches/UPS/etc. The top-level entity corresponds to the business location, followed by entities for Room, Rack, RackU, and finally device. Now, I know that primary keys are supposed to be unique, but is that unique over the entire database, or unique for the individual branch of the system?
By that I mean, there are one to many rooms in a location, and one to many racks in each room. do the primary keys for the racks in one room in a given location need to be unique from those of either another room in the same location, or even from a room in a different location?
I probably didn't describe this very well, so feel free to ask for clarifications on or at any point.