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Posted Apr 12, 2004

MDX Essentials: Basic Set Functions: The CrossJoin() Function

By William Pearson

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 Books Online and 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.

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