From a
conceptual point of view, aggregation is carried out on primary events thanks to the
definition of dimension attributes and hierarchies.
Definition 5: A dimension attribute is a property, with a finite domain, of
a dimension. Like dimensions, it is represented by a circle.
For instance, a product is described by its type, category, and brand; a customer, by
its city and its nation. The relationships between dimension attributes are expressed
by hierarchies.
Conceptual Modeling Solutions for the Data Warehouse
Copyright ?© 2007, Idea Group Inc. Copying or distributing in print or electronic forms without written permission
of Idea Group Inc. is prohibited.
Definition 6: A hierarchy is a directed tree, rooted in a dimension, whose
nodes are all the dimension attributes that describe that dimension, and
whose arcs model many-to-one associations between pairs of dimension
attributes. Arcs are graphically represented by straight lines.
Guideline 4: Hierarchies should reproduce the pattern of interattribute
functional dependencies expressed by the data source.
Hierarchies determine how primary events can be aggregated into secondary
events and selected significantly for the decision-making process. The dimension
in which a hierarchy is rooted defines its finest aggregation granularity, while the
other dimension attributes define progressively coarser granularities.
Pages:
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60