When we look at the cover of a book and read the title, the author
and the publisher then we are looking at a message and within the message
we see character strings that can be modelled as attributes of a book entity type
On reflection, we can distinguish the
as being particular to the book and the other two attributes
as not particular but, within the message, serving to identifying other entities.
These other attributes that are not primarily properties of the subject entity
in and by itself
but which are indicative of a relationship between the subject entity and
other entities we shall say are
Reference attributes are those attributes which appear in messages communicating
entities and singly or multiply
serve to identify entities other than the subject of the message.
Reference attributes are significant because they model the way that
relationships may be communicated in messages or, more specifically, represented
Understanding and modelling reference attributes is important because typically
or aquire models working from given examples and these are
represented to us in messages of various forms — all we have of entitites
is by way of messages and in these messages almost all we have of relationships
is reference attributes.
We have a notation for
reference attributes so that if required they can be shown on entity model
author and publisher as referential attributes
then we need introduce individual and publishing company
entity types, say, and introduce reference relationships as shown here:
We don't see all the possibilities in this single example but what we have done
- number the reference relationships R1, R2 and so on,
- depict the reference attributes as non-core,
- for each reference attribute identify, in parentheses3, the
reference relationship(s) it wholy or partly communicates.
Now let us suppose that a book is uniquely identified by the combination of
title and author
and not just by its title alone. To document this in the entity model
we show specify the author relationship by drawing a bar across the relationships
and see that now we underline the author name
attribute for now this
is an identifying attribute of book
A example is given in figure 35 giving a simple
model of molecular structure.
Model of molecular structure.