MDX Essentials: Basic Set Functions: The EXCEPT() Function - Page 5

January 12, 2004

Were the ALL flag not in place to override default behavior, elimination would be imposed within both sets prior to the evaluation and return of the difference. We will see an instance of the default behavior later.

3.  Execute the query by clicking the Run Query button in the toolbar.

The Results pane is populated, and the dataset shown in Illustration 4 appears.


Illustration 4: Query with EXCEPT() Function Results - Duplicates Retained Flag in Place

4.  Save the file as MDX15-4.

The behavior of the EXCEPT() function, with the ALL flag in place, can be explained in this way. Because it eliminates matching duplicates in first set first (that is, duplicates in the first set that match a member in the second set), Oregon (the only member in the second set that matches a pair of duplicates in the first set, is eliminated from the outset.

The non-matching duplicates (that is, duplicates in the first set that have no match in the second set) are retained. In the present case, that means that Washington and California, which exist as duplicates in the first set (they are included in [Store].[All Stores].[USA].Children, as well as appearing as [Store].[WA] and [Store].[CA], respectively), and are unmatched in the second set (where only [Store].[OR] appears), are retained as duplicates. Hence we see two of each of [Store].[WA] and [Store].[CA] appearing in the result dataset.

More succinctly, the rule for retaining duplicates is that matching duplicates in the first set are eliminated, and nonmatching duplicates are retained.

We can prove this to ourselves easily by making changes to the duplicates and re-executing the query, until we grasp the results that we obtain in each case. In addition, the remaining steps, which show the default operation of the EXCEPT() function, will demonstrate that duplicates are, indeed, eliminated when the default behavior is not overridden by the optional override flag.

5.  Remove the word "ALL" (along with the comma that precedes it) from the second set specification within the EXCEPT() function, as shown below:

{ EXCEPT

  ({[Store].[All Stores].[USA].Children, [Store].[OR], [Store].[CA], 

    [Store].[WA]} , {[Store].[OR]}, ALL )} ON ROWS

So that the EXCEPT() function appears as below:

{ EXCEPT

  ({[Store].[All Stores].[USA].Children, [Store].[OR], [Store].[CA], 

    [Store].[WA]} , {[Store].[OR]} )} ON ROWS

6.  Modify the comment line to read:

-- MDX15-5:  Tutorial Query Step 5

The Query pane appears as shown in Illustration 5.


Illustration 5: The Query with Modifications Circled

7.  Execute the query by clicking the Run Query button in the toolbar.

The Results pane is populated, and the dataset shown in Illustration 6 appears. We see clearly that the removal of the overriding flag results in the elimination of duplicates that were retained in our last step.


Illustration 6: EXCEPT() Function Results - Duplicates are Eliminated by Default

8.  Save the file as MDX15-5.

We have thus explored the use of the ALL flag within the EXCEPT() function to allow us to retain duplicates in a simple example. We have also exposed clearly the default behavior of the function, which is to eliminate the duplicates.

In conclusion, we can see that the EXCEPT() function provides us the significant capability of presenting the difference between two sets. Our use of the optional ALL flag within the syntax, can also afford us flexibility in the handling of duplicates in queries we construct that use this function - flexibility that can be paramount in meeting the needs of organizational information consumers.

Summary ...

In this lesson, we explored the useful EXCEPT() function, whose purpose is to return the difference between two sets. We have noted that, like the INTERSECT() function and other functions that we have explored in previous lessons, EXCEPT() allows us flexibility in duplicate handling, which is managed through the use of a flag which can be used to override the default elimination of duplicates.

The EXCEPT() function provides important capabilities within MDX, and is yet another means that MDX provides us to more efficiently access multidimensional data, as well as to write more concise code. In addition to discussing the purpose and operation of the EXCEPT() function, we focused on the treatment of duplicates by the function. We practiced the use of the function in general, through a multi-step practice example, then explored an additional example where we practiced the addition of the ALL flag, to override the EXCEPT() function's default duplicate handling. Throughout the multiple-step practice exercise, we discussed the results we obtained with each step's execution, remarking on the distinguishing characteristics of each.

» See All Articles by Columnist William E. Pearson, III

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MDX Essentials Series
The LEVEL_NUMBER Member Property
The LEVEL_UNIQUE_NAME Intrinsic Member Property
Intrinsic Member Properties: The HIERARCHY_UNIQUE_NAME Property
Intrinsic Member Properties: The DIMENSION_UNIQUE_NAME Property
Further Combination of BottomCount() with Other MDX Functions
Combine BottomCount() with Other MDX Functions to Add Sophistication
Basic Set Functions: The BottomCount() Function, Part I
Intrinsic Member Properties: The MEMBER_VALUE Property
Intrinsic Member Properties: The MEMBER_UNIQUE_NAME Property
Intrinsic Member Properties: The MEMBER_NAME Property
Intrinsic Member Properties: The MEMBER_KEY Property
Intrinsic Member Properties: The MEMBER_CAPTION Property
Set Functions: The StripCalculatedMembers() Function
Set Functions: The AddCalculatedMembers() Function
MDX Numeric Functions: The Min() Function
MDX Numeric Functions: The Max() Function
Set Functions: The .AllMembers Function
MDX Essentials: Set Functions: The MeasureGroupMeasures() Function
String Functions: The .Properties Function, Part II
String Functions: The .Properties Function
Logical Functions: IsGeneration(): Conditional Logic within Filter Expressions
MDX Scripting Statements: Introducing the Simple CASE Statement
Logical Functions: IsGeneration(): Conditional Logic within Calculations
Logical Functions: IsAncestor(): Conditional Logic within Filter Expressions
MDX Clauses and Keywords: Use HAVING to Filter an Axis
Logical Functions: IsAncestor(): Conditional Logic within Calculations
Logical Functions: IsSibling(): Conditional Logic within Filter Expressions
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MDX Operators: The IsLeaf() Operator: Conditional Logic within Filter Expressions
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MDX Numeric Functions: The .Ordinal Function
Other MDX Entities: Perspectives
MDX Operators: The IS Operator
MDX Set Functions: The Distinct() Function
MDX Set Functions: The ToggleDrillState() Function
Set Functions: The DrillUpLevel() Function
Set Functions: The DrillDownLevelTop() and DrillDownLevelBottom() Functions
MDX Set Functions: DrillDownLevel()
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MDX Essentials: String / Numeric Functions: The CoalesceEmpty() Function
MDX Essentials: Basic Set Functions: The TopCount() Function, Part II
MDX Essentials: Basic Set Functions: The TopCount() Function, Part I
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MDX Essentials: Set and String Functions: The GENERATE() Function
MDX Essentials: The CROSSJOIN() Function: Breaking Bottlenecks
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MDX Essentials: String / Numeric Functions: Introducing the IIF() Function
MDX Essentials: Logical Functions: The IsEmpty() Function
MDX Essentials: Basic Set Functions: The EXTRACT() Function
MDX Essentials: Numeric Functions: Introduction to the AVG() Function
MDX Essentials: Basic Member Functions: The .Item() Function
MDX Essentials: Basic Set Functions: Subset Functions: The Subset() Function
MDX Essentials: Basic Set Functions: Subset Functions: The Tail() Function
MDX Essentials: Basic Set Functions: Subset Functions: The Head() Function
MDX Essentials: Basic Set Functions: The CrossJoin() Function
MDX Essentials: Basic Numeric Functions: The Count() Function
MDX Essentials: Basic Set Functions: The Filter() Function
MDX Essentials: Basic Set Functions: The EXCEPT() Function
MDX Essentials: Basic Set Functions: The Intersect() Function
MDX Essentials: Basic Set Functions: The Union() Function
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MDX Time Series Functions, Part II: The OpeningPeriod () and ClosingPeriod() Functions
MDX Essentials - MDX Time Series Functions, Part I: PeriodsToDate() and Kindred Functions
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MDX Essentials: Member Functions: More "Family" Functions
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