Basic Set Functions: The BottomCount() Function, Part I

About the Series …

This article is a member 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 for getting the most out of the lessons
included, please see my first article, MDX at First Glance: Introduction to MDX Essentials.

Note: Current updates are assumed for MSSQL
Server, MSSQL Server Analysis Services, and the related Books
Online and Samples.

Overview

In this article, we will examine the BottomCount() function,
whose greatest utility lies in its capability to enable us to isolate worst /
lowest performers from among numbers (even hundreds, thousands or more) of
fellow members. This ranking capability is critical in data analysis and
decision support scenarios: In many business situations, we seek to report upon
“worst” (or simply “lowest / least”) performers for various reasons. BottomCount()
facilitates our doing so, allowing us to sort on a numeric value expression
that we can provide. We can tell the function how many “bottom” members
we wish to retrieve (say, the “bottom ten,” or the “bottom twenty”), for a
“custom-fit” approach, that matches our needs precisely.

In
this article, we will introduce and overview the BottomCount() function.
We will first comment upon the general operation of BottomCount(), and
then we will:

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

The BottomCount() Function

Introduction

According
to the Analysis Services Books Online, the BottomCount() function
“sorts a set in ascending order, and returns
the specified number of tuples in the specified set with the lowest values.” The
BottomCount() function stands out as an excellent general example of the
potential power of MDX. We specify three parameters, a set expression,
a count, and a numeric expression (typically an MDX expression of
cell coordinates that return a number), and BottomCount() returns the
number of bottom performers (or “bottom” / “least”, in effect), based upon our
input.

As
we shall see, BottomCount() sorts the set we specify by the numeric
expression we provide (if we provide one) within the function, thus
breaking the natural hierarchy of the set. The basis of sorting by BottomCount()
closely resembles that used by the TopCount() function. If a
numeric expression is not specified, the function returns the set of members in
natural order, without any sorting, behaving like the Tail() function.

We will examine the syntax for the BottomCount()
function, and then look at its behavior based upon input we might provide. Moreover,
we will undertake practice examples constructed to support hypothetical
business needs that illustrate uses for the function. This will allow us to
activate what we explore in the Discussion and Syntax
sections, by getting some hands-on exposure in creating expressions that
leverage the function. Finally, our next article will build upon the basics we
expose here, and leverage BottomCount() within more sophisticated
examples of business usage.

Discussion

To restate our initial explanation of its operation, the BottomCount()
function sorts a set based upon a numerical expression we
provide, and then picks the bottom (whatever number we provide in the Count
parameter of the function) items in that set, based upon the rank order
established by the function. The frequent requirement to examine a subset
of tuples at the bottom of the general set is easily answered by the BottomCount()
“sort, and then pick this number of members, from the bottom up” approach.

The BottomCount() function can, of course, be used
for a great deal more than the support of simple listings with “least values.”
When we couple it with other functions, we can leverage BottomCount() to
deliver a wide range of analysis and reporting utility. As in so many cases
with the Microsoft integrated business intelligence solution, consisting of MSSQL
Server, Analysis Services and Reporting Services, this
function, leveraged from within the Analysis Services layer, can be
extended to support capabilities and attributes in the Reporting Services
layer. Knowing “where to put the intelligence” among the various layers is
critical to optimization, in many cases. For more of my observations on this
subject, see Multi-Layered
Business Intelligence Solutions … Require Multi-Layered Architects.

Let’s look at some syntax illustrations to further clarify
the operation of BottomCount().

Syntax

Syntactically, the set
upon which we seek to perform the BottomCount() operation is specified
within the parentheses to the right of BottomCount, a common arrangement
within MDX functions, as we have seen in many of our articles. The syntax is
shown in the following string.

BottomCount(<< Set >>, << Count >> [,<< Numeric Expression >>])

We follow <<Set>>, the set
specification, with a comma, which is followed by <<Count>>, the numeric
expression we provide to dictate the number of “bottom” tuples we wish for
the function to return. <<Count>> is, in turn, followed by a numeric
expression whereby we specify the “sort by” criteria. As we have
mentioned, BottomCount() always breaks the natural hierarchy of the set
– due to the fact that the set is sorted upon the third argument we
provide as <<Numeric Expression>>, before returning the bottom
(specified number of) tuples from that sort.

The items returned by BottomCount()
are limited to the << Count >> input that we provide,
even in the case of “ties” in the <<Numeric Expression>> value.
This may be important to consider when evaluating the data retrieved by the
function, in addition to behavior, already mentioned above, of the function
when no sort criteria (in the form, again, of the <<Numeric
Expression>>) is provided.

We will practice some uses of the BottomCount() function
in the section that follows.

Practice

Preparation

To reinforce our understanding of the basics we have covered
so far, we will first use the BottomCount() function in a simple
scenario, to illustrate its operation as the primary focus. We will do so in a
backdrop that places BottomCount() within the context of meeting a
business need for a group of hypothetical information consumers – with
requirements similar to those we might encounter in our respective daily
environments. The intent, of course, is to demonstrate the operation of the BottomCount()
function in a straightforward, memorable manner.

We will turn to the SQL Server Management Studio as a
platform from which to construct and execute the MDX we examine, and to view
the results datasets we obtain. If you do not know how to access the SQL
Server Management Studio in preparation for using it to query an Analysis
Services cube (we will be using the sample Adventure Works cube in
the Adventure Works DW Analysis Services database), please
perform the steps of the following procedure, located in the References
section of my articles index:

This procedure will take us through opening a new Query
pane, upon which we can create our first query within the section that follows.