A Quick Introduction to Attributes
As stored within information systems then the individually represented items — the
names, colours, quantities, the monetary amounts, the dates, etc — in the language
of entity modelling, are said to be the values of attributes. Thus an actual name
is said to the value of a ‘name’ attribute of a ‘person’ entity.
We may express that information about a person is communicated or stored as
message with two components name and data of birth we might say that a ‘person’ entity
may be attributed a ‘name’ and a ‘date of birth’
on show an entity model diagram representing type ‘person’
with ‘name’ and ‘date of birth’ attribute annotations, like this:
Alternatively to say
that date of birth is optional
we may use a circle in place of the square:
To show that the name attribute within a message is the identifying attribute we underline
the annotation in the diagram:
If it is nessary to give both a person's name and their data of birth to uniquely identify
them then we underline
both of these attributes on the diagram:
Generally, systems will hold and communicate many different attributes of each type
of entity and
these attributes are shown beneath the identifying attributes:
It is clear that computer programs are effective only in so long as the data items
they manipulate are intended and understood as attributes of subject entities. It
follows that to have an effective information system we must first have agreed types
of subject entity and also what may be attributed to entities of these types. In
we agree the data content of the program or system i.e.
its subject matter.
In a message about a person two or more phone numbers may be communicated
but it is a rule of entity modelling that for a single attribute an entity may only
be attributed a single value.
For this reason if a person can have multiple phone numbers then ‘phone_number’ is
not an attribute of a ‘person’ entity type per se but an attribute of a ‘phone’ entity type that
stands in relation to (is owned by) the
A More Considered View - Attributes as a Shorthand for Certain Relationships
Entity types, for practical purposes, are only as useful as our ability to communicate
and to communicate such an instance, i.e. an entity, we need communicate its relationships
with other entitites.
A subset of these relationships serve to identify the entity within the chosen perspective.
These other entitites, those we might wish to communicate relationships with, need
in turn be identified
which in turn implies communicating their relationships with other entities.
This has to end somewhere else there is an infinite regress. Therefore for practical
in computer science there need be
types of entity which, a priori, we are given a means of communicating
so that instances of these givens will be the atoms in all communications of
In practice, as used in computer science, but not in theory, the atoms are certain
that are domain non-specific such as the
types character, character string, number
both integer or floating point,
date and boolean
and these usually occurring many times in relation to each domain-specific entity
shown in the example in figure 1.
It profits us little to show these relationships with the atomic types on the entity
diagram and therefore a different notation is used as illustrated in
Because relationships with the atomic types are sufficiently
different to other relationships (as we shall see later),
there is a third term, attribute used for them instead in place of the
To conclude, in theory, an attribute is a kind of relationship however
to a practicing entity modeller this is not the way we think, we think instead of
as a named property of a type of entity,
it can be optional or mandatory and it can have a type which is
one of character, string
(meaning character string), integer, float
The attributes are shown itemised in the boxes, nested or otherwise,
representing the entity types on which they occur.
See figure 2 for an illustration.
How a model might look with types on the diagram for character string
The same model as figure 1 but now using the attribute notation.
Relationships to the universal types string
are represented as attributes. Optional attributes
are depicted with a leading circle; these correspond with the cardinality zero-or-one
relationships in the previous diagram whilst mandatory attributes
— those shown with a leading square — replace relationships of cardinality exactly-one
. Identifying attributes, i.e. those replacing identifying relationships, are distinguished
by having their names underlined.
Example - Chemical Formulae
In chemistry the chemical formula of a compound is one of the ways of expressing information
about the numbers of the different kinds of atoms that constitute a particular molecular
For example water, famously, has formula H2O.
Consider the information given in (9)
Each bullet describes a distinct chemical compound giving its molar mass, some of the names that it is known by
and its chemical formula.
- aspirin (also know as acetylsalicylic acid) has the formula C9H8O4;its molar mass is 180.16
- common salt (also known as sodium chloride or halite) has the formula NaCl; its molar
mass is 58.44
- water (also know as oxidane) has the formula H2O; its molar mass is 18.01
Analysing this information we understand that each chemical compound may have many
aliased names, and that
its formula describes many occuring types of element. These considerations lead us
to this entity model in figure 3
and also to the tabular
display in table 1.
A model of chemical compound information.
Table of chemical compounds.
The rows of this table are messages communicating entitites:
the columns correspond to attributes,
- the rows correspond to subject entities,
each cell presents the value
of an attribute of a subject entity,
- some rows have other rows nested within them; these are subordinate messages and representing
entities in the sense defined by the composition structure in the model.
Example - Chemical Elements
In any given situation, getting to the right blend of entity types, attributes and
may be an iterative process as demonstrated in this next example in which
we explore two entries for chemical elements from a scientific dictionary1:
oxygen (Chem.). A nonmetalic element, symbol O, at. no. 8, r.a.m. 15.994, valency 2.
It is a colourless, odourless gas which supports combustion and is essential for the
respiration of most forms of life. M.p. -218℃, b.p.-183℃,
density 1.42904 g/dm3 at s.t.p., formula O2.
An unstable form is ozone, O3. Oxygen is the most abundant element, etc.
chlorine (Chem.). Element, symbol Cl, at.no. 17, r.a.m. 35.453, valencies 1-,3+, 5+, 7+, m.p.
-101℃, b.p. -34.6℃. The second halogen, chlorine is a geenish yellow gas, with an
irritating smell etc.
From these entries, and with some expansion of abbreviations, I first surmise that
each element has a name, a symbol,
an atomic number, a relative atomic mass, one or more valencies, a melting point,
a boiling point, and,
optionally maybe, a density. I am left unsure whether all elements have a formulae,
or whether they may have
forms which may have formulae which is probably closer to the truth. I provisionally
model this situation on an
entity model diagram like so:
Reading the entry for sulphur, I find this preliminary model to be inadequate:
sulphur (Chem.). A nonmetalic element occurring in several allotropic forms. Symbol S, at.
no. 16, r.a.m. 32.06, valencies 2,4,6. Rhombic(α-) sulphur is a lemon yellow powder;
m.p. 112.8 ℃, rel. d. 2.07. Monoclinic (β-) sulphur has a deeper colour than the rhombic
form; m.p. 119 ℃, rel.d. 1.96, b.p. 444.6C. Chemically, sulphur resembles oxygen etc.
From this entry, I learn that different forms of sulphur have different melting points
and that my provisional model has miss-positioned the ‘melting point’ attribute; it
turns out that chemical elements,
in and by themsleves, cannot be attributed melting points.
After a little further research I learn that these forms taken by chemical elements
technically speaking, allotropes
and that it is these
that have melting-points, boiling points and densities.
I reach this model:
In tabular form the dictionary entries for oxygen, chlorine and sulphur can be
structured like this2 :
Example - Attributes in a Model of a Personal Computer
In the example in figure 4 based on the operational state of a personal
computer all files and folders are shown as
having ‘name’ and ‘date modified’ attributes. In addition files, but not folders,
and ‘content’ attributes.
Model of a Personal Computer.
In this example I have based the attributes of the process entity type on the data
that I typically see in the
table of processes that I can view through Task Manager, reached, for example, through
Ctrl-Alt-Delete on my current Windows system.
I have omitted the name of the executable program because this is the name of a referenced
something or other (the program); instead,
I have represented this by a reference relationship — this being the core concept
— rather than by an attribute
which is a derivative of this core concept.
Attributes are represented on entity model diagrams in place of relationships with
types character,string, number etc. They are represented as mandatory or optional. If they are identifying then
they are shown underlined.
Entities are communicated, in tabular or other forms, by communicating the values
of their attributes.
Models should distinguish between core attributes and those derivative of the core;
we describe this further in the next section.