The numeric measures in a fact table fall into three categories.
The most flexible and useful facts are fully additive; additive measures can be
summed across any of the dimensions associated with the fact table. Semi-additive
measures can be summed across some dimensions, but not all; balance amounts are
common semi-additive facts because they are additive across all dimensions
except time. Finally, some measures are completely non-additive, such as
ratios. A good approach for non-additive facts is, where possible, to store the
fully additive components of the non-additive measure and sum these components
into the final answer set before calculating the final non-additive fact.
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