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Chapter 3
details translate directly into facts and degenerate dimensions (denoted as DD)
within the fact table. Chapter 9 covers instances where physical how dimensions
are needed.
DD : Degenerate Dimension. A dimensional attribute stored in a fact table.
Documenting Dimensions
Dimensions are
modeled using
BEAM✲ tables
Dimensions are documented by taking each event detail (that has additional
attributes), one at a time, starting with the event subject and modeling it as a
BEAM ✲ (example data) table. Figure 3-1 shows how CUSTOMER — the subject of
CUSTOMER ORDERS — is used to define a dimension of the same name. Note that
dimensions are singular whereas events are plural.
Dimension Subject
The event detail
becomes the
subject of the
dimension
Figure 3-1
Modeling the
Customer
dimension
The event detail also becomes the first attribute of the dimension table — its subject.
This mandatory attribute of the dimension is denoted by the code MD. You should
ask the stakeholders for a suitable subject name. Typically, a dimension’s subject is
its “Name” attribute, such as CUSTOMER NAME, PRODUCT NAME, or
EMPLOYEE NAME. After you have a name for the attribute you populate it with
the unique examples from the event table: notice in Figure 3-1 that the repeated
customer names have been removed.