Why do I need type 3 and 6 SCDs when I can implement type 7?

Hi everyone,

I normally implement type 1 and 2 SCDs, but I am faced with some new business requirements where they want to see what the historical numbers look like using today's view of the world. I am reading about the various SCD types in the Data Warehouse Toolkit, 3rd Edition.

Type 3, 6 and 7 sound like they will do the job. Type 7 looks really flexible. When compared to type 3, I do not have to add new fields to my dimension table with each passing year. When compared to type 6, it easily extends to support many attributes where the historical and current data needs to be maintained.

Why do I need type 3 and 6 SCDs when I can implement type 7? I am assuming it boils down to a design decision that looks at the nature of the business requirements, the ability for analysts/end-users to accept a more complex model, etc. I am trying to understand the factors I should be considering before making that design decision.

Regards,
James

You don't "need" any of them. They are simply variations on types 1, 2 & 3. Choose the one that fits your problem.

Thanks for the reply ngalemmo. Agreed, type 7 is just a variation of types 1, 2, and 3. My question is really about the implementation. Looking at the data models in Ch. 5 of the Data Warehouse Toolkit, 3rd Ed., it appears that the type 7 data model offers more flexibility to deal with future change requests than the data models for type 3 and type 6.

I can see that a more complex data model can cause end-users confusion and defeats one of the core principals of making the DW/BI system easy for end-users to understand. That said, I give most of my users a set of views or cubes to use and not direct access to the DW. The exceptions being analysts who have a good grasps of data concepts.

It looks like that when I am faced with situation where I need to report numbers based on the world view today or the historical world view, then I should just implement type 7. My gut tells me I may be missing something. I guess I am looking to find out if I am missing or cases where this could come back to bite me.

Regards,
James