Functions such as NonEmpty(), Filter() and Exists() must use a numeric expression

Many of the MDX functions, such as NonEmpty(), Filter() and Exists(), seem to be easy enough to use, but they present quite a challenge for people with T-SQL background. The common theme among them is that they all claim to return dimension members, such as a list (or a set of tuples in MDX lingo) of countries, but they must use a fact measure (a numeric expression in another word) as the condition, such as the sales amount must be more than $10K. We are just not used to using fact measures.

Check out this post for an example of the NonEmpty() function.

MDX #1 – Why should we always provide set_expression2 in NonEmpty() function

Use Except() function to leave out a dimension member

In many situations, we just simply want to leave out a dimension member, such as one of the Product Categories, no matter what the sales amount is.

Run this simple query, and you will get the Order Quantity for all the Product Categories for each month.

SELECT    [Date].[Month of Year].Children  ON COLUMNS,
               [Product].[Product Categories].[All].Children ON ROWS
FROM      [Adventure Works]
WHERE   ([Measures].[Order Quantity])

What if I simply just want to leave out the Components in my query? Components are not the “final” products. Clothing, Bikes and Accessories are. So I have quite a good reason to leave it out of the query.

Now run this query with the help of the Except() function. Components is no longer in the result.

SELECT    [Date].[Month of Year].Children  ON COLUMNS,
       Except(
            [Product].[Product Categories].[All].Children ,
            [Product].[Product Categories].[Components]
        ) ON ROWS
FROM    [Adventure Works]
WHERE   ([Measures].[Order Quantity])

The two sets must be from the same hierarchy and at the same level

The basic syntax of the Except() function is:

Except(Set_Expression1, Set_Expression2)

It returns SET 1, minus the SET 2. Please make sure that both sets are:

1) from the same hierarchy, and

2) at the same level

If you want to leave out Brakes, the following query will not work, because the two sets in the Except() function is not at the same level.

(The following would not work.)

SELECT    [Date].[Month of Year].Children  ON COLUMNS,
        Except(
           [Product].[Product Categories].[All].Children ,
            [Product].[Product Categories].[Subcategory].[Brakes]
        ) ON ROWS
FROM    [Adventure Works]
WHERE   ([Measures].[Order Quantity])

This query should work because both sets are at the Subcategory level.

SELECT    [Date].[Month of Year].Children  ON COLUMNS,
        Except(
            [Product].[Product Categories].[Subcategory].members ,
           [Product].[Product Categories].[Subcategory].[Brakes]
        ) ON ROWS
FROM    [Adventure Works]
WHERE   ([Measures].[Order Quantity])

The Except() function does not involve any numeric expressions

So hopefully, it’s easier for people to understand with T-SQL background.