Dimension is a term in data management and data warehousing that refers to logical groupings of data such as geographical location, customer information, or product information. Slowly Changing Dimensions (SCDs) are dimensions that have data that changes slowly, rather than changing on a time-based, regular schedule.
For example, you may have a dimension in your database that tracks the sales records of your company’s salespeople. Creating sales reports seems simple enough, until a salesperson is transferred from one regional office to another. How do you record such a change in your sales dimension?
You could sum or average the sales by salesperson, but if you use that to compare the performance of salesmen, that might give misleading information. If the salesperson that was transferred used to work in a hot market where sales were easy, and now works in a market where sales are infrequent, her totals will look much stronger than the other salespeople in her new region, even if they are just as good. Or you could create a second salesperson record and treat the transferred person as a new sales person, but that creates problems also.
Dealing with these issues involves SCD management methodologies referred to as Type 0 through 6. Type 6 SCDs are also sometimes called Hybrid SCDs.
In most of our projects we use Type 2 i.e., SCD2.
For more info, please look into http://en.wikipedia.org/wiki/Slowly_changing_dimension
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