MDX Essentials: Basic Set Functions: Subset Functions: The Head() Function
May 10, 2004
About the Series ...
This is the nineteenth article of the series, MDX Essentials. The series is designed to provide hands-on application of the fundamentals of the Multidimensional Expressions (MDX) language, with each tutorial progressively adding features designed to meet specific real-world needs.
For more information about the series in general, as well as the software and system requirements needed for getting the most out of the lessons included, please see the first article, MDX at First Glance: Introduction to MDX Essentials.
Note: Service Pack 3 updates are assumed for MSSQL Server 2000, MSSQL Server 2000 Analysis Services, and the related Books Online and Samples.
What We Accomplished in our Last Article
In the last article of the series, Basic Set Functions: The CrossJoin() Function, we explored the highly popular CrossJoin() function. We discussed the general purpose of the function, to allow us to produce all possible combinations of two sets, and cited its common use to "collapse" two or more dimensions onto a single axis for purposes of presenting multidimensional data in a two-dimensional matrix.
We began by introducing CrossJoin(), commenting upon its operation and touching upon performance considerations at a general level. We examined the syntax surrounding the function, and then began practice exercises to illustrate the operation of the function, within hypothetical scenarios where we described a business need. We then set about solving the need with a straightforward MDX query that exploited the CrossJoin() function. Finally, we briefly discussed the results datasets we obtained in the practice examples.
In this lesson, we will begin a "triptych" of articles that expose set functions that deal specifically with subsets - that is, each function returns a subset of a larger set as part of its operation. We will introduce the Head() function in this article, then follow it with the Tail() function in the subsequent article. The last of the three subset functions that we will investigate will be, unsurprisingly, the Subset() function. These three functions have much in common with regard to usage and operation, and covering them in this fashion will allow us to more finely distinguish among them, to exploit the attributes we can leverage to meet specific business needs.
The general purpose of the Head() function is to return the first specified number of elements in a set. We will introduce the Head() function, commenting upon its operation, and then we will: