The Role of Role Playing Dimensions


Perhaps you have heard the term Role Playing Dimension in regards to PowerPivot and/or Analysis Services cubes. This terms refers to the ability of one dimension to be linked to more than one fact in the fact table. Now strictly speaking, there is still only a single primary link between the dimension and the fact table and that link is used as the default link when aggregating data by that dimension. However, some dimensions, such as date dimension, often can be used against multiple fields in the fact table. For my example, I will use the Adventure Works DW data set that I use in many of my SQL Saturday presentations since it represents a typical sales database. Another good sample database is the Contoso dataset. Using the reseller sales table as my fact table (FactResellerSales), I can proceed to pull into my Excel PowerPivot table this fact table along with the dimensions for Product, Product Category, Product SubCategory, and Date. Notice that there is at least one relationship between each table and one other. This primary relationship is represented by a solid line in the figure below. However, there are three relationships between the FactResellerSales table and the DimDate table. One of the relationships is represented by a solid line and the other two are represented by dotted lines. Back in the source database, these three relationships are represented by regular indexes between these two tables. There is no indication to define which relationship is more important than the other. So how does PowerPivot select which one is the primary relationship between the two tables? The best I can figure out is that it is based on the order of the three different date keys in the fact table. In this case the OrderDateKey appears first and is therefore selected as the primary or Active relationship.

Looking at the raw sales data for a moment, I can see that the dates in the OrderDateKey, DueDateKey, and ShipDateKey are different with the order date occurring first followed by the due date and then the ship date. This would make sense in the real world. Unfortunately, if I did not have the ability to use role playing dimensions for each of these dates, I would either have to deal with only a single relationship between the date dimension table and one of these dates such as the order date or have multiple date dimension tables, one for each relationship. If I only used a single date dimension table, I would have to pick one date in my sales table to relate that dimension. Then any sum I calculated such as the sum of the order amounts, the sum of the ship amounts, or the sum of the due amounts would be associated with a single date such as the order date even though the due date may not occur until the next month or even next year. Unfortunately, this sample data does not show many such cases. However, I will show you one case at the end of the blog.

Our pivot table would then look something like this:

However, that would not be correct since we did not ship the items on the same day as the order nor was the amount due on the same day as the order. Therefore, we need another way to relate these two table when performing a summing calculation. That other way includes the use of the USERRELATIONSHIP() function which lets us define for the purposes of a single calculation which relationship we want to use. For example, to calculate the sum of the amounts ordered, we can sum the Sales Amount field by the OrderDateKey field as shown in the following equation:

When placed in our measure area of the FactResellerSales table, I can see that this calculation returns a value of over 80 million. (I will format as currency this in a moment.)

Similarly, the Shipped Amount can be calculated by summing the Sales Amount using the ShipDateKey as shown in this equation:

Finally the Due Amount can be calculated with the following equation using the DueDateKey field.

After formatting the measures as currency (which saves me time by not having to format this data in the resulting pivot tables separately), the measures appear as this:

Now I can proceed to build the pivot table by clicking on the PivotTable button in the Home ribbon and then selecting

PivotTable from the dropdown.

Excel prompts me to create the PivotTable in either a new worksheet or an existing worksheet. I will select a new worksheet here.

After clicking OK, Excel creates my empty PivotTable and opens the Field List to let me begin defining my PivotTable.

I choose a very simple layout with my calculated measures going across the top of the table and a hierarchy of time coming down the rows of the table.

By default when displaying the names of the months, Excel displays them alphabetically. Typically this will not please your manager. Therefore using a technique I discussed in an earlier blog, I define a column that contains the month number as the column by which I want to sort the column names.

Keep in mind that you only have to do this once. All subsequent PivotTables and PivotCharts will use the sort order assigned to the column EnglishMonthName without my having to do anything else.

Now the months appear in the correct order.

But more importantly for this demonstration, if I drill down into any of the months, I will see the sum of the order amounts, shipped amounts, and due amounts correctly summed by actual dates. Some positions will naturally be empty if there was no order, ship, or due activity for that day.

As promised, the following figure shows an example where orders were taken on two different days within the month but shipping and due dates only occurred once. The point being that the individual sums are correct for the days in which they appear.

Looking at the orders in April of the above figure, you may question the math saying that $2,204,542 plus $82 is $2,204,624, not $2,204,623. This is the result of rounding each individual amounts to whole dollars and is not an indication that Excel cannot perform math. All sums are calculated on the actual dollar amounts and then rounded.

So this is one simple example of how to use role playing dimensions. Dimensions other than dates can serve as role playing dimensions, but most people will encounter the need for role players when working with dates.

C’ya next time.

Advertisements

One comment on “The Role of Role Playing Dimensions

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s