MDX Essentials: Basic Set Functions: Subset Functions: The Subset() Function - Page 4

July 12, 2004

The Results pane is populated, and the dataset depicted in Illustration 3 appears.

We have used the Order() function, with the BDESC keyword in place, to obtain the sorted core dataset that the Marketing department wants to see. This allows the information consumers to narrow even further their requirements for a recurring report on the promotions activity by gender. In our next section, we will use the Subset() function to provide for these narrowed, more informed requirements.

NOTE: For details concerning our use of the Order() function above, see my article Basic Set Functions: The Order() Function.

Limiting the Initial Dataset with the Subset() Function

Having provided the Marketing team with a "big picture" idea of promotions activity from the Sales cube, we have equipped them to ask for data within a narrower scope, to eliminate outliers such as promotions that fall below thresholds of interest for various reasons. For purposes of our practice example, we will say that the Marketing information consumers respond to our sorted results dataset within a short period, as we expected, requesting that we provide the report, exactly as it currently appears, on a monthly basis, but that the No Promotion group be excluded (it is of little value in the current context of specific promotion analysis), and that only the top twelve (on the basis of female patronage) promotions be presented in the recurring report.

There are numerous ways to approach this with MDX functions, but we know that Subset() will handle the requirement, particularly in a scenario where we have a sort in place for the dimension member under examination, females.

Let's use the Subset() function to meet the business requirement with precision.

-- MDX021-3, Use of Subset() Function within the Ordered Query

{[Gender].Members} ON COLUMNS,

SUBSET(

[Measures].[Unit Sales]), BDESC)} ON ROWS

1, 12)

The Query pane appears as shown in Illustration 4.

Note that we set "1" as «Start», because, conveniently enough, we wish to exclude the "0" position (the No Promotions line item) anyway, based upon the request of the Marketing consumers who have defined the business requirement. We set "12" as the «Count», because the same information consumers have requested that we provide the metrics for the range of the top twelve promotions in the final version of this recurring report.

The Results pane is populated, and the dataset shown in Illustration 5 appears.

We have thus provided the Marketing department with the requested analytical data. Because we have built in, via the Order() function, the automatic sorting on the criteria requested, we can be confident that any future generation of the data via this query will provide the appropriate selection, together with the order that reflects the sort of the core dataset. Should the consumers return with a request to change the number of promotions to which they want to narrow their focus, we can accomplish this with a simple adjustment to the «Count» specification within the Subset() function we have placed into our query.

Summary ...

This article served as the conclusion of a group of three articles surrounding subset-related functions. We introduced the Subset() function, whose general purpose is to return a specified number of elements in a set, beginning at a point in the set that we designate via the «Start» value, and extending for a range of «Count» tuples. We commented upon the operation of the function, and then examined its syntax.

We undertook a multi-step practice example whereby we created a core query, then limited the results that the query returned through the use of the Subset() function, within the context of meeting an illustrative business requirement. We demonstrated the manner in which the Subset() function uses the «Start» and «Count» values we input to generate the precise results that we wish to obtain. We briefly discussed the results dataset we obtained with the Subset() function, together with other surrounding considerations. Throughout our examination of the Subset() function, we compared and contrasted the Subset() and the Head() and Tail() functions, from the perspective of usage and operation, in order to finely distinguish among them for the particular characteristics we need to meet specific business needs.

» See All Articles by Columnist William E. Pearson, III

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