Dimensions and Dimension Tables
A dimension answers the what portion of a question, or how a consumer wants to
analyze data. A dimension is normally stored in a single table, although this is not
always the case, as will be discussed shortly.
Most dimensions are hierarchical. This means that they contain higher-level items
which break down into lower-level items. Time is a perfect example: Years break down
into Quarters, which break down into Months, which break down into Days, and so
on. Note that there are no hard and fast rules here as to what the hierarchy should be.
Some companies would use Years to Half Years to Quarters to Months to Days. Some
solutions would require Weeks which might or might not roll up into Months.
Hierarchical structures occur in most dimensions. The previous example included a
Location dimension in which Plants broke down into Assembly Lines. This Location
dimension might actually include Country, then State/Province, then City, and finally
Plant and Assembly Line. Figure 3-2 shows what the hierarchical dimensions might
look like for the Time and Location dimensions for this simple example.
C h a p t e r 3 : D a t a W a r e h o u s i n g a n d B u s i n e s s I n t e l l i g e n c e 35
So far only three terms have been discussed: facts, dimensions, and hierarchical
dimensions. It??™s important to discuss two other terms: levels and members. A level is
a position within a hierarchical dimension. In other words, the Time dimension has a
Year level, a Quarter level, a Month level, and a Day level.
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