Conceptual Modeling Solutions for the Data Warehouse
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Consider the fact schema modeling the sales of books in a library, represented in
Figure 5, whose dimensions are date and book. Users will probably be interested
in analyzing sales for each book author; on the other hand, since some books have
two or more authors, the relationship between book and author must be modeled
as a multiple arc.
Guideline.6: In presence of many-to-many associations, summarizability
is no longer guaranteed, unless the multiple arc is properly weighted.
Multiple arcs should be used sparingly since, in ROLAP logical design,
they require complex solutions.
Summarizability is the property of correcting summarizing measures along hierarchies
(Lenz & Shoshani, 1997). Weights restore summarizability, but their introduction
is artificial in several cases; for instance, in the book sales fact, each author
of a multiauthored book should be assigned a normalized weight expressing her
???contribution??? to the book.
Shared.Hierarchies
Sometimes, large portions of hierarchies are replicated twice or more in the same
fact schema. A typical example is the temporal hierarchy: a fact frequently has more
than one dimension of type date, with different semantics, and it may be useful to
define on each of them a temporal hierarchy month-week-year.
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