when to use a seperate month dimension?

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when to use a seperate month dimension?

Post  salaman on Mon May 16, 2011 4:29 am


looking at various Kimball examples I notice that he often uses a seperate month dimension when there is also a day dimension that contains a month_key.

Could someone explain the value of using a seperate month dimension and if there are any specific situations in which it would be preferrable (or not) to use one?




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Re: when to use a seperate month dimension?

Post  ngalemmo on Mon May 16, 2011 10:36 am

The primary purpose is for clarity in the model. There is no ambiguity as to the grain of the measures if the fact table uses a month dimension. It doesn't affect conformance if the attribute values in the month dimension are identical to corresponding attributes in the date dimension.

Personally, I prefer to use a single date dimension table with additional natural keys for month, quarter and year. These NK columns would usually be null except for designated rows used to represent a particular period (such as the last day of each month to represent month values). I would then use role names in the fact table to clarify the time grain (such as month_date_key). For presentation purposes, I may sometimes create views (month view, quarter view, etc...) if it helps clarify things.

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