Temporal changes: An E-R diagram usually models the state of an enterprise
at a point in time. Suppose we wish to track temporal changes, that is,
changes to data over time. For example, Zhang may have been a student
between 1 September 2005 31 May 2009, while Shankar may have had instructor
Einstein as advisor from 31 May 2008 to 5 December 2008, and
again from 1 June 2009 to 5 January 2010. Similarly, attribute values of an
entity or relationship, such as title and credits of course, salary, or even name
of instructor, and tot cred of student, can change over time.
One way to model temporal changes is as follows.We define a new data
type called valid_time, which is a time-interval, or a set of time-intervals.
We then associate a valid_time attribute with each entity and relationship,
recording the time periods during which the entity or relationship is valid.
The end-time of an interval can be infinity; for example, if Shankar became
a student on 2 September 2008, and is still a student, we can represent
the end-time of the valid_time interval as infinity for the Shankar entity.
Similarly, we model attributes that can change over time as a set of values,
each with its own valid_time.
a. Draw an E-R diagram with the student and instructor entities, and
the advisor relationship, with the above extensions to track temporal
changes.
b. Convert the above E-R diagram into a set of relations.
