Improving Expressive Power in Modeling Data Warehouse and OLAP Applications - Evolving Application Domains of Data Warehousing and Mining - page 22

Figure 3. A conceptual representation of the logical schema from Figure 2

Color

Category

Group

SalesOrder
Number

Product
colors

Product SubCategory

Country Region

subordinate

Due date
Order date
Ship date

Reseller
sales

Date

Employee

Time Assignation

/SalesAmount
OrderQuan tity
UnitPrice
Max(TotalCost)

supervisor

+

Calendar month Fiscal quarter Department

+

Reseller

Calendar quarter Fiscal year

Location

StateProvince

Calendar year

x x

City CountryRegion

to this work and we omit all attributes since they
are the same as in Figure 2.
A schema consists of a set of dimensions and
a set of fact relationships. A dimension is an
abstract concept that allows grouping data that
share a common semantic meaning. A dimension
is composed of a level or a set of hierarchies.
A level corresponds to an entity type in the ER
model. It describes a set of real-world concepts
that have similar characteristics, e.g., the Product
level in Figure 3. Instances of a level are called
members . A level has a set of attributes that
describes the characteristics of their members
and one or several keys that identify uniquely
the members of a level. These attributes are not
shown in Figure 1, however, they can be seen in
Figure 6a, e.g., the attribute Country name is a key
attribute for the Country level, while Population
is its descriptive attribute.
A hierarchy comprises several related levels,
which represent different data granularities, i.e.,
levels of detail. For example, the Employee level

contains specific information about employees,
while the Department level may be used to ana-
lyze employees from the higher perspective of
the department to which they are assigned. Given
two related levels of a hierarchy, the lower level
is called the child and the higher level is called
the parent . Thus, the relationships composing
hierarchies are called parent-child relationships .
Since these relationships are used only for travers-
ing from one level to the next, they are represented
by a line to simplify the notation.
Parent-child relationships are characterized
by cardinalities , indicating the minimum and the
maximum number of members in one level that
can be related to a member in another level. We
use different symbols for indicating cardinalities:
represents (1,n), implies (0,n),
means (1,1), and indicates (0,1).
For example, in Figure 3 the child level Product
is related to the parent level SubCategory with a
one-to-many cardinality, which means that every
product belongs to only one sub-category and that