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MS SQL

Posted Aug 7, 2007

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

By William Pearson

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 Logical Functions: IsGeneration(): Conditional Logic within Calculations, another article within my MDX Essentials series, we introduced the IsGeneration() function from the perspective of its use within a calculation. We discussed the straightforward purpose of this logical function: IsGeneration() provides us a means of testing whether or not a specified member exists within a specified generation. As a part of introducing the function, we noted that the effective use of IsGeneration() as a testing mechanism is dependent upon a good understanding of the manner with which generation numbers are assigned within Analysis Services. For this reason, we overviewed the generation concepts, within our preliminary commentary surrounding the function, in Logical Functions: IsGeneration(): Conditional Logic within Calculations.

In this article, we will examine IsGeneration(), once again as a conditional logic modifier, but within the context of a filter. Combining IsGeneration() 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 enhance our overall awareness of the potential of IsGeneration(). From the perspective of the use of the IsGeneration() 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 IsGeneration() Function

Introduction

As we related in Logical Functions: IsGeneration(): Conditional Logic within Calculations, the Books Online tell us that the IsGeneration() function “returns true if the member indicated ... is in the generation specified....” Otherwise, the function returns false. Also, if the member we specify evaluates to an empty member, the IsGeneration() function returns false. A Boolean value of “True” is returned if the member to which the function is applied resides within the generation number specified by the Numeric Expression we provide, and “False” is returned if it does not (or if the member expression we supply identifies an empty member). In its capacity, as a logical function, to “test” the nature / status of a member, IsGeneration() can be 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. Moreover, as we shall see in this article, IsGeneration() can be employed more directly within a filter expression we place into the specification of an axis within an MDX query.

We will examine in detail the syntax for the IsGeneration() 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 of the basic options that IsGeneration() can offer the knowledgeable user. Our current examination will focus upon the use of IsGeneration() within the context of a filter. Hands-on practice with IsGeneration(), where we will create queries that employ the function, will help us to activate what we have learned in the Discussion and Syntax sections.

NOTE: For more detail surrounding the Filter() function, see Basic Set Functions: The Filter() Function, a member of my Database Journal MDX Essentials series.

Discussion

To restate our initial description of its operation, IsGeneration() returns “True” if a specified Member Expression is the number of “steps” specified (by the Numeric Expression) away from the leaf level of the containing hierarchy. Alternatively, “False” is returned if the specified Member Expression is not the specified number of steps away, or if the Member Expression evaluates to an empty member. We can use IsGeneration() 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 IsGeneration() with other MDX functions can help us to leverage its power beyond what we might otherwise achieve in attempts to use it in standalone fashion.

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

Syntax

Let’s review the syntax involved with employing the IsGeneration() function. We employ IsGeneration() by specifying the Member Expression (the member which we are testing as to “generation member status”) and the Numeric Expression (the generation number in relation to which we are testing the Member Expression) within parentheses to the immediate right of the function. The function takes the Member Expression and Numeric Expression thus appended as its arguments, and returns True if the member denoted by the Member Expression exists within the specified generation number (Numeric Expression) of the hierarchy within which it resides (or, in other words, if the Member Expression lies the number of steps specified by the Numeric Expression from the leaf level of the containing hierarchy).

If the member specified by the Member Expression is evaluated as an empty member, or if the Member Expression does not exist within the generation number specified by Numeric Expression, then a False is returned, as we have noted.

The general syntax is shown in the following string:

IsGeneration(Member_Expression, Numeric_Expression)

As we noted earlier, employing IsGeneration() is as straightforward, in the mechanical sense, as working with most of the MDX logical functions, assuming that we have an adequate grasp of the meaning of generation, (a topic we discussed in depth in Logical Functions: IsGeneration(): Conditional Logic within Calculations). As we learned, we simply place the Member Expression and Numeric Expression, 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 Date (with a hierarchy of Calendar), the following pseudo-expression:

IsGeneration([Date].[Calendar].CurrentMember, 4)

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

  • CY 2001
  • CY 2002
  • CY 2003
  • CY 2004

Each of the listed members is a “resident” of generation number 4 of the Date.Calendar hierarchy – which appears among the other levels as annotated in Illustration 1.


Illustration 1: Generation Number 4 among the Other Generations of the Hierarchy ...

NOTE: For information on several of the “relative” functions, of which .CurrentMember (used in the pseudo-expression above) is an example, see my article MDX Member Functions: "Relative" Member Functions, within the Database Journal MDX Essentials series.

We will practice some uses of the IsGeneration() function, within the context of filtering, in the section that follows.



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