Are dates degenerated dimensions?

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Are dates degenerated dimensions?

Post  evgeninikolov on Tue Jul 17, 2012 11:20 am

Hi,

Am I allowed to look at date dimensions as like a degenerated dimensions or do I need always to have a surrogate key for them?

Regards,

Evgeni

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Re: Are dates degenerated dimensions?

Post  BoxesAndLines on Tue Jul 17, 2012 1:09 pm

A degenerate dimension is a source system identifier with no describing attributes used to group facts. That is not the purpose of a date. Add the foreign key.
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Re: Are dates degenerated dimensions?

Post  ngalemmo on Tue Jul 17, 2012 4:22 pm

evgeninikolov wrote:Hi,

Am I allowed to look at date dimensions as like a degenerated dimensions or do I need always to have a surrogate key for them?

Regards,

Evgeni

What does your date dimension look like? It should contain a lot of things, such as month labels, day of week, fiscal calendar data, and so on. The question is, does the date you are trying to store need these attributes? If it does, then store a FK to the date dimension. If it is something like an effective date that would only ever be used as a date to filter selections, then you can just store the date itself as a degenerate dimension.
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Re: Are dates degenerated dimensions?

Post  evgeninikolov on Tue Jul 17, 2012 4:35 pm

ngalemmo wrote:
What does your date dimension look like? It should contain a lot of things, such as month labels, day of week, fiscal calendar data, and so on. The question is, does the date you are trying to store need these attributes? If it does, then store a FK to the date dimension.

Yeah, it is a full blown date dimension :-( And since the fiscal year is different than the calendar year I need those attributes. OK, so the decision is taken! I think that the main argument is that there should be a special values for invalid or infinite dates as AFAIK null values and outer joins are not permited in the dimensional modelling.

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