Logical Functions: IsAncestor(): Conditional Logic within Calculations

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 lesson, we will expose another logical function which we can use for
testing a member or level at which a cell is being calculated,
the IsAncestor() function. The general purpose of IsAncestor() is
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.)

The IsAncestor()
function, like other logical functions and operators,
evaluates values and returns a Boolean value. The utility of IsAncestor() becomes
clear when we realize the capability that it gives us to determine the “position,”
together with the relationship to progenitors, of a member within a dimensional
hierarchy. IsAncestor() more specifically allows us to test whether a member
is an ancestor of another member that we specify within the dimension to which
it belongs.

Similar to IsLeaf(), IsSibling(), IsChild(),
IsGeneration(), and other MDX functions, IsAncestor() can best be
employed to apply conditional logic within a couple of primary ways: as a
component within a calculation, and as a component within a filter
expression. In this article, we will concentrate upon IsAncestor() from
the perspective of its use within a calculation. We will discuss the straightforward
purpose of the function, to ascertain (and indicate) whether a member
is the ancestor of another specified member; the manner in which IsAncestor()
manages to do this; and ways we can leverage the function to support
effective conditional logic to meet various business needs within our
own environments.

Along
with an introduction to the IsAncestor() function, this lesson will
include:

• an examination of the syntax surrounding the function;

• illustrative examples of uses of the function within practice
  exercises;

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

The IsAncestor() Function

Introduction

According to the Books
Online, 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 (to which I will refer as the “primary member expression”
throughout this article) is an ancestor of the second specified member
(the “secondary member expression”); 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 simple, hypothetical
business needs that illustrate a use for the function. This will afford us an
opportunity to explore some the basic options that IsAncestor() can
offer the knowledgeable user. 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 in an attempt to
use it in standalone fashion.

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

Syntax

Syntactically, we employ the IsAncestor()
function by specifying 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, in the parentheses to the right of the function.
As an example, 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.