Making Data Models Readable - High-Performance Web Databases: Design, Development, and Deployment - page 239

an automated tool), there is no rationale for entities' relative positions on
a diagram. They wind up wherever it is convenient. In some cases, people
will rearrange entities to minimize the crossing of lines, but this adds no
semantic meaning to the picture.
This has the effect that as you look at a model drawing, you have no way
to get a hold of its overall meaning. You see a random collection of boxes
and lines. There is nothing in its overall shape to tell you anything about it.
A very powerful convention for positioning entities is to force all rela-
tionships to point in the same direction. Your author follows the “dead
crow” rule, that all crow's foot toes should be be pointing either up or to
the left. This has the effect of placing the reference entities — the entities
describing tangible things in the business — on the lower right, and the
transaction entities — those describing what the enterprise does — on the
upper left. Others reverse the convention, pointing crow's foot toes south
and east. Whichever convention is followed, anyone looking at a model fol-
lowing the dead crow convention can now quickly determine the subject
(products, laboratory tests, etc.) and the elements of interest.
Too Many Symbols
The simplest way to improve the appearance of a model is to limit the
number of figures (any two-dimensional graphic object) on any one draw-
ing. Twenty seems to be about the maximum that is possible without mak-
ing a drawing completely unreadable, and ten to fifteen is better.
The number of relationship lines present is less significant, and this will
necessarily be a function of the number of things to be connected. The
number of line segments can be reduced as well, by eliminating “elbows” in
the line. A bent line after all is graphically two lines.
Too Many Symbol Types
Clearly the more different kinds of figures that are on the page, the more
confusing the result will be. The question is, how many are needed? Each
different kind of symbol is something new for the viewer to learn, and there-
fore is one more obstacle to understanding. One of the reasons models
often become cumbersome is that too much is attempted with them. There
is a limit to the amount of information that can be conveyed on one page.
As a minimum, the model needs symbols for:
• Entities
• The existence of relationships between pairs of entities
• Optionality: Must an occurrence of one entity have a corresponding
occurrence of another, or not?
• Cardinality: May an occurrence of one entity have more than one cor-
responding occurrence of another, or not?