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Posted Dec 8, 2003

MDX Essentials: Basic Set Functions: The Intersect() Function - Page 4

By William Pearson

We have now become familiar with the data populating the two sets that we wish to intersect. This should make the operation of the Intersect() function clearer in our next step.

12.  Select File --> New from the main menu.

A new, blank query pane appears.

13.  Type the following query into the Query pane:


-- MDX14-3:  Tutorial Query Step 3
SELECT
{[Measures].[Number of Employees]} ON COLUMNS,
INTERSECT
({([Store].[All Stores].[USA].[OR]), 
    ([Store].[All Stores].[USA].[CA]), 
    ([Store].[All Stores].[USA].[WA])} , 
   {([Store].[All Stores].[USA].[OR]), 
   ([Store].[All Stores].[USA].[WA])} )
  ON ROWS
FROM HR

The purpose of this query is to return the intersection of the pair of sets we have examined individually, using the Intersect() function.

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

The Results pane is populated, and the dataset shown in Illustration 3 appears.


Illustration 3: Our First Intersect() Function Results

Finally, we see the effects of the Intersect() function. Note that the resulting dataset is identical to the last query, shown in Illustration 2 above. This "second half" of the pair of sets upon which we intended to perform the Intersect() function represents a complete subset of the first (the other "partner" in the Intersect() function. By its very nature, the subset is a duplicate of the first set, and thus forms the "intersecting" members. We are thus presented with the identical result in Step 3 as in Step 2, of our exercise above - this is the set that the two initial sets held "in common."

15.  Save the file as MDX14-3.

Next, we will examine the operation of the function from the standpoint of how it handles duplicate members in the sets it is being asked to intersect.

Dealing with Duplication

Let's establish a scenario whereby we can explore the handling of duplicates by the Intersect() function. We will accomplish this by creating a query whose objective is to intersect sets that we know to contain a duplicate member, as we did in Lesson 13 with the Union() function, and will thus venture out of a completely "real world" scenario. This is to make the operation of the function clear, however, and we can certainly rely upon the fact that there are many situations in the business environment where dealing with duplicates is a fact of life.

1.  Select File --> New from the main menu.

The blank query pane appears.

2.  Type the following query into the Query pane:


-- MDX14-4:  Tutorial Query Step 4
SELECT
{[Measures].[Number of Employees]} ON COLUMNS,
INTERSECT
({([Store].[All Stores].[USA].[OR].Children),([Store].[All Stores].[USA].[OR].[Portland])} , 
   {([Store].[All Stores].[USA].[OR].Children),([Store].[All Stores].[USA].[OR].[Portland])} )ON ROWS
FROM HR

The Intersect() function is called into action this time to perform an intersect between two sets that we know to contain an identical member. We are asking that the complete set of the Oregon city-children, which includes the city of Portland, be combined with the set of the single city of Portland in the initial set specification, then that the resulting set be intersected with a set identical to itself. (Were the flag not in place to override default behavior, elimination would be imposed within the first set prior to its intersection to the second set.) The purpose, again, is to illustrate clearly the default behavior of the Intersect() function, with regard to the handling of such duplicates.

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: Intersect() Function Results - Duplicates Eliminated

4.  Save the file as MDX14-4.

The behavior of the Intersect() function is demonstrated in its elimination of the duplicate members - a characteristic that is the default of the syntax with which we have been working.

We can prove this to ourselves by overriding the default behavior, and using the ALL flag to allow the duplicate to be retained. To do so, we will make an adjustment to the query we created in Step 4 above.

5.  Insert the word "ALL" into the second set specification within the Intersect() function, that is:

   {([Store].[All Stores].[USA].[OR].Children),([Store].[All Stores].[USA].[OR].[Portland])} )ON ROWS

between the right-most curly brace and the closing parenthesis, as shown:

   {([Store].[All Stores].[USA].[OR].Children),([Store].[All Stores].[USA].[OR].[Portland])}, ALL )ON ROWS

6.  Modify the comment line to read:

-- MDX14-5:  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 the instance of the duplicate set is retained, thanks to the insertion of the ALL flag into our Intersect() function.


Illustration 6: Intersect() Function Results - Duplicates Retained via the ALL Flag

8.  Save the file as MDX14-5.

And so we see how to retain duplicates in a simple example that illustrates the influence of the ALL flag upon the Intersect() function. In conclusion, we can see that the Intersect() function provides us the significant capability of performing intersections of 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.

Summary ...

In this lesson, we explored the commonly used Intersect() function, whose purpose is to return the intersection, or the common members, of two sets. We discussed the Intersect() function in general, and emphasized its capabilities within MDX, and its usefulness within our analysis toolsets.

In addition to discussing the purpose and operation of the Intersect() function, we focused on the treatment of duplicates by the function. We practiced the use of the function in general, then with an example of the addition of the ALL flag, to override the Intersect() 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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