Data Modelling Examples
Personal Computer Example
In the following example based on the personal computer all files and folders are
shown as
having ‘name’ and ‘date modified’ attributes. In addition files, but not folders,
have ‘size’
and ‘content’ attributes:
Chemical Elements
In any given situation, getting to the right blend of entity types, attributes and
relationships
may be an iterative process as demonstrated in this next example in
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/dm
3
at s.t.p., formula O
2
. An unstable form is ozone, O
3
. 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.
Reading this third 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
are,
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 :
|
name
|
symbol
|
atomic no
|
r.a.m.
|
valency
|
allotrope
|
|
|
|
|
oxygen
|
O
|
8
|
15.994
|
|
|
dioxygen
|
-218
|
-183
|
??
|
|
Ozone
|
???
|
???
|
???
|
|
|
chlorine
|
Cl
|
17
|
35.453
|
|
|
|
suplhur
|
S
|
16
|
32.06
|
|
|
Rhombic
|
112.8
|
???
|
2.07
|
|
Monoclinic
|
119
|
444.6
|
1.96
|
|
In this table:
-
the columns correspond to attributes — each column heading is the name of an attribute,
- the rows correspond to subject entities,
-
each cell presents the value
of an attribute of a subject entity,
- some rows have other rows inside them representing the parts of subject
entities i.e. the compositional substructure.
The overall structure of each row, considered as a messages, we will represent
by these message structure definitions:
| element
=>
|
name,
|
|
symbol,
|
|
atom no,
|
|
relative atomic mass,
|
|
valency*,
|
|
allotrope* |
| allotrope
=>
|
name,
|
|
mp,
|
|
bp |
