### 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

**and**

*Books Online***.**

*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.

### Introduction

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:

- Examine the syntax surrounding the function;
- Undertake illustrative examples of the uses of the function in practice exercises;
- Briefly discuss the results datasets we obtain in the practice examples.