MDX Scripting

Multidimensional Expressions (MDX) is the query language for Microsoft SQL Server Analysis Services. MDX is used to retrieve data from cubes, define calculations, and perform complex analytical operations. This section covers the fundamentals of MDX scripting within the context of multidimensional modeling.

Understanding MDX Script Structure

An MDX script is a set of statements that are executed in order to define calculations, session variables, and other objects within a cube. The script is typically associated with a cube or a database. Key components include:

• DIMENSION STATEMENT: Used to define or modify dimension properties.
• CALCULATION STATEMENT: Used to define calculated members, sets, and measures.
• SET VARIABLE STATEMENT: Used to assign values to session variables.
• DEFINE MEMBER STATEMENT: Used to define calculated members.
• ASSIGN MEMBER STATEMENT: Used to assign values to existing members or calculated members.

Core MDX Concepts for Scripting

Calculated Members

Calculated members are dynamically computed members that can be defined within a cube or a specific scope. They allow you to create new measures or dimensions based on existing data without altering the underlying data source.

Syntax:

-- Define a calculated member in the Measures dimension
CALCULATE MEMBER [Measures].[Average Sales] AS
    [Measures].[Internet Sales Amount] / [Measures].[Internet Order Quantity]

-- Define a calculated member with specific scope
WITH MEMBER Measures.AverageSalesByRegion AS
    ([Measures].[Internet Sales Amount] / [Measures].[Internet Order Quantity]) ON COLUMNS

Scope and Context

MDX operates within a specific context. The context of a query or a calculation determines which data is visible and how it is aggregated. The SCOPE statement is crucial for defining specific regions of the cube where calculations should apply.

SCOPE([Date].[Calendar Year].&[2023]);
    // Calculations within this scope
    ([Measures].[Sales Target], [Geography].[Country].&[USA]) = 100000;
END SCOPE;

Session Variables

Session variables allow you to store temporary values that can be used throughout an MDX script or multiple queries within a session. This is useful for parameterizing calculations or storing intermediate results.

-- Set a session variable
SET @SalesThreshold = 5000;

-- Use the session variable in a calculation
CALCULATE MEMBER [Measures].[High Value Sales] AS
    SUM(
        {
            ([Measures].[Sales Amount] > @SalesThreshold)
        },
        [Measures].[Sales Amount]
    );

Common MDX Scripting Statements and Functions

Statements

• CALCULATE: Evaluates expressions and applies them to the current context.
• WITH: Used to define calculated members, sets, or variables for a specific query.
• SET: Assigns a value to a session variable.
• DIMENSION: Modifies properties of a dimension.
• CONSTRAINT: Limits the scope of a calculation.

Functions

MDX provides a rich set of functions for manipulating data, performing calculations, and navigating the cube structure.

• Aggregation Functions: SUM, AVG, COUNT, MAX, MIN.
• Set Functions: FILTER, ORDER, UNION, INTERSECT.
• Navigation Functions: MEMBER, CHILDREN, PARENT, ANCESTOR.
• String Functions: LEFT, RIGHT, SUBSTRING.
• Numeric Functions: ABS, ROUND, SQRT.

Best Practices for MDX Scripting

• Readability: Use clear naming conventions and indentation.
• Performance: Avoid complex calculations within cell-by-cell processing. Optimize the use of SCOPE and WITH.
• Modularity: Break down complex calculations into smaller, reusable components.
• Testing: Thoroughly test your MDX scripts with various scenarios.

Example: Defining a KPI (Key Performance Indicator)

This example shows how to define a calculated member representing Sales Variance Percentage.

-- Define the measure for Sales Variance Percentage
CALCULATE MEMBER [Measures].[Sales Variance %] AS
    IIF(
        [Measures].[Budget Sales Amount] = 0,
        NULL,
        ([Measures].[Sales Amount] - [Measures].[Budget Sales Amount]) / [Measures].[Budget Sales Amount]
    );

-- Format the calculated member as a percentage
ALTER MEMBER [Measures].[Sales Variance %]
    .FORMAT_STRING = 'Percent';

This MDX script defines a new measure, [Sales Variance %], which calculates the difference between actual sales and budget sales, then divides by budget sales. The IIF function handles division by zero. Finally, ALTER MEMBER is used to set the formatting to display the result as a percentage.