Databases Reference
In-Depth Information
Milan
N
W
E
S
Pisa
600 km
People
350 km
> 2,000,000
Pescara
Rome
From 1,000,000
to 2,000,000
From 200,000
to 1,000,000
< 500,000
220 km
200 km
Naples
Figure 2.2:
Correct and complete representation of data in Table
2.1
.
system is based on a knowledge base containing different kinds of rules: 1) visual rules that char-
acterize the different kinds of visual symbols that are associated with the different kinds of visual
symbols; 2) data rules that specify the characteristics of the data model, the database schema, and
the database instances; 3) mapping rules that specify the link between data and visual elements (e.g.,
entities are represented as rectangles, Person is a red rectangle, John is a small red rectangle); and
4) perceptual rules that describe how the user perceives a visual symbol (i.e., a line, a geometric
figure, an icon, etc.), relationships between symbols (i.e., the mutual placements of two figures on
the plane), and what the perceptual effects are of relevant visual attributes such as color, texture, etc.
Associating an effective visual representation to any large data set is crucial to allow different
kinds of non-technical users to easily grasp the database information content. However, as opposed
to the case of data models, the creation suitable visual models is not straightforward, and it is still
an open problem how to do this, in general.
Despite the theory that aims to provide a correct and complete visual representation, existing
VQSs implement much more specific visual representations.
Form-based
representations are the simplest way to provide users with friendly interfaces for
data manipulation. They are very common as application or system interfaces to relational databases
where the forms are actually a visualization of the tables. In query formulation, prototypical forms are
