Microsoft Office Tutorials and References
In Depth Information
understanding Physical and Logical Data Models
Now that you have taken a first look at what a data model is, you need to learn the difference
between the physical and the logical data model. To help you do that, we are going to show
an example of a physical data model, describe its limitations and issues, and introduce the new
concept of logical data models.
Let us recall the data model you used to analyze products, categories, and subcategories in
Chapter 2. There, you had to load into PowerPivot three different tables and connect them
by means of the chained relationship shown in Figure 4-14.
FIguRE 4-14 The relationships among products, subcategories, and categories.
Probably you don’t remember the difficulty we had with this data model, so it might be worth
recalling it. When it came to using a pivot table to query this data, you had to query three
different tables. Then, because you would normally think of categories and subcategories as
simple attributes of the product table, you used the RELATED function to add the new columns
ProductCategory and ProductSubcategory to the product table. Then you managed to hide
the two lookup tables so that the final data model to query was much more intuitive. In
Chapter 2, we called that data model your first; now we are going to describe in much more
detail what you did there.