Hi!

I would like to state the following question...

How should we select actors for a Use Case Diagram? For example see the following case:

Tracel and Tourism Ltd (TAT advertise and manage properties for rent in popular holiday resorts arount the Eastern Mediterranean and black sea coasts. Properties can be registered with TAT by their owners or by an agent who acts on behalf of one or more owners.When property owners or agants reg their property with TAT they must provide info about their property, TAT staff will use this info to complete an Accomodation Form and save it into a File.

Are the actors:

1. Owner
2. Agent
3. Tat Staff

or only Tat Staff as they interact with the system where Owners / Agents provide information to the Tat Staff who then complete a form (so they interact with the system)?

UML is just a diagramming convention: you can make agents and owners into "actors" if it makes for a useful diagram.

What has this to do with "database concepts and design"?

Well from UML you can understand the functonality of a system, then generate an ERD and design your DB...

You can't generate an ERD from a Use Case diagram, unless UML has changed a lot since I last looked?