### About the Series ...

This is the eighteenth 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 systems 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 Numeric Functions: The Count() Function**,
we took a break from purely set-related functions to focus on the numerical

**Count()**function, as it is

*applied*to sets. We introduced the

**Count()**function, discussing its purpose, to return the number of cells in a specified set. We then exposed options within the syntax for overriding the default behavior of the function with regard to its handling of empty cells.

Along
with an introduction to the purpose of the **Count()** function, we examined
the syntaxes surrounding the function. We next undertook illustrative examples
where we used **Count()**, both with and without the **EXCLUDEEMPTY**
keyword, to meet a hypothetical business need. Finally, throughout the steps of
our examples, we discussed the results we obtained using MDX.

As a
part of the practice exercises in our last article, we performed a query using
the **CrossJoin()** function. Our purpose was to use **CrossJoin()** as
a "proofing" procedure, to ensure the accuracy and completeness of
the results we obtained within our practice with the **Count()** function.
As part of this exercise, we were afforded a preview of the **CrossJoin()** function,
upon which we will focus this article.

### Introduction

In this
lesson, we will expose another highly popular function in the MDX toolset, the **CrossJoin()**
function. The general purpose of the **CrossJoin()** function is to allow us
to produce all combinations of two sets. A primary driver for wanting to do
this is to "collapse" two (or perhaps more) dimensions onto a single
axis.

Uses
of the **CrossJoin()** function can be quite sophisticated, and, as we shall
learn, its use should be tempered with an understanding of its potential impact
upon performance. We will introduce the function, commenting upon its
operation and touching upon performance considerations at a general level, and
then we will:

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