The advantage of a Type II dimension is clear: history is preserved. Companies
will always be able to track when the change occurred so that prior sales will still
roll up to the proper manager. Product changes will be evident because a change will
start a new record with a new start date.
The disadvantage of a Type II dimension is also clear: it??™s complicated. It makes
storing, retrieving, and summing data much harder. Changes require an update to the
existing record (to set the end date) and the insertion of a new record with a new version
number and the start date. Taking multiple records that represent a single product or
employee and making them appear as one to the end user can be a challenge.
There are other ways of handling slowly changing dimensions. There are Type
III, and modifications of Type II and Type III. The actual mechanisms are beyond
the scope of this book, as the goal is to show how to consume the data once the
warehouse is built. Still, slowly changing dimensions are introduced here because
many readers will be involved in the decision of how to store dimension data and
track history, and an understanding of the tradeoffs is important.
C h a p t e r 3 : D a t a W a r e h o u s i n g a n d B u s i n e s s I n t e l l i g e n c e 39
Parent-Child Dimensions A special type of dimension that is encountered frequently
is the parent-child dimension, or p-c dimension for short. Product is an example of a
normal, or non p-c, dimension.
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