Logical Functions: IsAncestor(): Conditional Logic within Filter Expressions

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 IsAncestor():
Conditional Logic within Calculations, another article
within my MDX Essentials
series, we introduced the IsAncestor() function from the perspective of
its use within a calculation. We discussed the straightforward purpose of this
logical function, to return whether or not a specified member is an
ancestor of another member we specify (By “ancestor,” of course, we mean a
member from which the specified member is descended within a dimensional
hierarchy.) We discussed the manner in which IsAncestor() manages to do
this, and ways we can leverage the operator to support effective conditional
logic to meet various business needs within our own environments.

In
this article, we will examine IsAncestor(),
once again as a
conditional logic modifier, but within the context of a filter.
Combining IsAncestor() with the MDX Filter()
function is another way we commonly see it in action in the business
environment, and our exposure to the practical aspects of its employment in
this way will serve to round out our overall awareness of the potential of IsAncestor(). From the perspective of the use
of the IsAncestor()
function in combination with Filter(),
this article will include:

• A review of the general syntax surrounding the function;

• Illustrative examples of uses of the function in practice
  exercises;

• A brief discussion of the MDX results obtained within each of the
  practice examples.

The IsAncestor() Function

Introduction

As we related in IsAncestor():
Conditional Logic within Calculations, the Books
Online tell us that the IsAncestor() function “returns whether a
specified member is an ancestor of another specified member.” A Boolean value of “True” is returned if
the member expression to which the function is applied is an ancestor of
the second specified member; otherwise IsAncestor() returns “False.” In its capacity, as a logical function, to “test”
the nature / status of a member, IsAncestor() is often employed in
conjunction with the IIF() function to conditionally drive the return of
data, such as a member or members, or values, based upon the relationship between
members as ancestor / descendant.

We will examine in detail the
syntax for the IsAncestor() function after our customary overview in the
Discussion section that follows. Following that, we will conduct
practice examples within a couple of scenarios constructed to support
hypothetical business needs that illustrate uses for the function. This will
afford us an opportunity to explore some the basic options that IsAncestor()
can offer the knowledgeable user. Our current examination will focus upon
the use of IsAncestor() within the context of a filter. Hands-on practice with IsAncestor(),
where we will create queries that employ the function, will help us to activate
what we have learned in the Discussion and Syntax
sections.

Discussion

To restate our initial description
of its operation, IsAncestor() returns “True” if a specified
member expression represents an ancestor of another member (that is,
lies between the secondary member and the top / “All”
dimensional level) that we specify within a given use of the function;
otherwise, the function returns “False.” We can use IsAncestor() to
apply conditional logic, based upon the location and / or existence of
members. As we have noted to be the case with most MDX functions, pairing IsAncestor()
with other MDX functions can help us to leverage its power much further
than we might do in an attempt to use it in standalone fashion.

Let’s look at syntax specifics to further clarify the
operation of IsAncestor() .

Syntax

To review the syntax involved
with employing the IsAncestor() function, we specify the primary
member expression (the member which we are testing as to “ancestor status”)
and the secondary member expression (the member in relation to which
we are testing the primary member expression) within parentheses to the
immediate right of the function. The function takes the member expressions
thus appended to it as its arguments, and returns True if the member
denoted by the primary member expression is an ancestor of
the secondary member expression (or, in other words, if the primary
member lies somewhere between the secondary member and the “top” of
the dimensional hierarchy).

If the member specified by the primary
member expression is not an ancestor of the secondary
member (or if the primary member and the secondary member belong
to different dimensions) a False is returned, as we might expect.

The general syntax is shown in
the following string:

IsAncestor(Primary_Member_Expression, Secondary_Member_Expression)

Employing IsAncestor(), like
most of the MDX logical functions, is, in the mechanical sense,
straightforward. As we have noted, we simply place the primary and secondary
member expressions, respectively, within the parentheses to the right of
the function. To use the example from our syntax discussion in IsAncestor(): Conditional Logic within Calculations, within a query executed against the sample Adventure
Works cube, for the dimension named Geography (with a hierarchy of
the same name), the following pseudo-expression,

IsAncestor([Geography].[Geography].[State-Province].[South Australia],
      [Geography].[Geography].CurrentMember) 

returns True for the current
member of the Geography dimension / Geography hierarchy for
each of the following:

• Cloverdale
  • 6105
• Findon
  • 5023
• Perth
  • 6006

Each of the listed members is a descendant of South
Australia in the cube, as shown in Illustration 1.

Depending upon the structure of the query (and specifically
upon whether the syntax defining axes, etc., eliminates nulls), if members of
other dimensions, or members of levels higher than South Australia
within the Geography hierarchy, were returned in, say, the row axis of
the dataset, their values would be null.

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