Relational database primary key requirements 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.
For each relation (ie. table) the primary key value must be unique and immutable (ie. it doesn't change).

You're likely to have a relation for location, one for rooms, one for racks. The primary key values for each of them must all be unique, so two racks in different rooms would not have the same PK value. However you could have the same PK value for a rack and a location, because they are different relations. There would be no significance to the values being the same or different across relations.