Measures are included
in the bottom section of the fact.
For instance, each invoice line is measured by the number of units sold, the price
per unit, the net amount, and so forth. The reason why measures should be numerical
is that they are used for computations. A fact may also have no measures, if
the only interesting thing to be recorded is the occurrence of events; in this case
the fact scheme is said to be empty and is typically queried to count the events that
occurred.
Definition 3: A dimension is a fact property with a finite domain and
describes one of its analysis coordinates. The set of dimensions of a fact
determines its finest representation granularity. Graphically, dimensions
are represented as circles attached to the fact by straight lines.
Typical dimensions for the invoice fact are product, customer, agent, and date.
R zz
Copyright ?© 2007, Idea Group Inc. Copying or distributing in print or electronic forms without written permission of
Idea Group Inc. is prohibited.
Guideline.2: At least one of the dimensions of the fact should represent
time, at any granularity.
The relationship between measures and dimensions is expressed, at the instance
level, by the concept of event.
Definition 4: A primary event is an occurrence of a fact, and is identi-
fied by a tuple of values, one for each dimension. Each primary event is
described by one value for each measure.
Pages:
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58