Databases Reference
In-Depth Information
We say that there is a change in the state of the domain at time t if the
entities or relationships that exist at t are different from those existing at the
previous time. In other words, a state change is a change in the population or
one or more entity or relationship types between two states: the new state
(corresponding to t ), and the old state (corresponding to t
1).
Any change in the population of an entity (relationship) type can
always be decomposed into a set of one or more elementary changes of the
following types:
-
Insertion of entity (relationship). This change happens when there is
an entity (relationship) in the new state that did not exist in the old
state.
·
Deletion of entity (relationship). This change happens when there
was an entity (relationship) in the old state that does not exist in the
new state.
·
The causes of the changes are the events [3, 12]. A domain does not change
its state if no event happens. An event is any circumstance that happens at a
given instant and whose effect is a change in the domain state. Normally,
these circumstances are actions (or decisions) performed by human beings
that act on a domain (e.g., hiring an employee or making a move in a chess
game), but they also may be the result of physical processes (e.g., dropping
some amount of liquid into a tank or the rising of the sun).
It is usually assumed that events are instantaneous, that is, they do not
have duration. It is also assumed that an event causes a transition in the
domain, from an old state to a new one, without any noticeable intermediate
state. In many cases, those assumptions do not pose any particular problems.
For example, the process of hiring a new employee takes some time, but it is
likely that we are interested only in the outcome of that process: From that
moment on, the person will be an employee, which he or she was not at the
previous time.
In some cases, however, events have duration. To handle those cases in
conceptual models that require instantaneous events, it may be necessary to
refine the conceptual schema of the domain or the event itself. For example,
assume the domain includes the relationship type Is at between persons
and places. In principle, it seems natural to consider that persons are at some
place at any moment. Let us consider now the event corresponding to the
move of a person from an origin to a target. If we assume that the event
is instantaneous, then the person will continue to be at some place at any
