In this appendix, we discuss some aspects about the problem of identifying actual entities in the real world within the proposed database instance.
First of all, it was said that our two employees are identified by ED and DI, respectively. Are those a sort of surrogates or not?
If they are, how can we use them in formulating queries and in results of queries? (They do not appear in the proposed taxonomy).
For instance, in the given examples, employee names are allowed to change over time and it is assumed that the system is responsible to mantain the links between the tuples referring to the same real-world entity. However there is no mean to distinguish in the query result between a person who has changed name from two different persons. If we consider the query Q 2.1.1 ``Find the names of employees that have been in a department named Toy for a shorter period than has DI ", how can we establish that Ed and Edward are two different names of the same person, rather than being the names of two different persons?
A similar problem occurs if the user wants to know something about the whole story of an employee formulating a query knowing only his name at some point in time. It would be reasonable that the system will retrieve all the tuples with the specified employee name and the tuples with the previous/successive names of the same employee. But in this way we do not allow the system to retrieve the tuples which refer to the name the user has explicitly specified, only.
We think that it would be useful to support both kinds of queries:
The problem is that we need a way to distinguish between the identifier ED (whatever it is) and the string ``Ed" representing the name of an employee at a given time, both in query formulation and query results.