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MDX Calculations in Analysis Services Multidimensional Modeling

Multidimensional Expressions (MDX) is the query language for Microsoft SQL Server Analysis Services (SSAS) multidimensional databases. Calculations are a fundamental part of building a robust and insightful SSAS cube. They allow you to define complex business logic, derive new metrics from existing data, and present information in a meaningful way to end-users.

This section delves into creating and managing calculations within your Analysis Services multidimensional models using MDX.

Understanding Calculation Concepts

  • Measures: The raw numerical data you want to analyze (e.g., Sales Amount, Quantity Sold).
  • Calculated Members: Members defined using MDX that are computed dynamically during query execution. They can represent new business metrics, forecasts, or comparisons.
  • Calculated Measures: Similar to calculated members, but typically defined at the measure level within a cube.
  • Named Sets: Predefined sets of tuples that can be used in MDX queries, often for performance or ease of use.
  • SCOPES: Statements that specify how calculations apply to different parts of the cube.

Key MDX Functions for Calculations

MDX provides a rich set of functions for manipulating data and defining calculations. Some of the most common include:

  • SUM(): Aggregates values.
  • AVG(): Calculates the average.
  • COUNT(): Counts the number of members or tuples.
  • IIF(): Conditional logic (If-Then-Else).
  • CASE statements: More complex conditional logic.
  • YTD(), QTD(), MTD(): Time-intelligence functions for year-to-date, quarter-to-date, and month-to-date calculations.
  • ParallelPeriod(): Compares the current period with the same period in a prior year.
  • Ancestor(), Parent(), Children(): Navigation functions within the dimension hierarchy.
  • StrToMember(), StrToTuple(), StrToValue(): Functions for dynamic member/tuple/value construction.

Creating Calculated Members

Calculated members are defined using the WITH MEMBER statement in an MDX query or within the cube definition itself in Visual Studio.

Syntax Example (in an MDX Query):

WITH MEMBER [Measures].[Gross Profit] AS '[Measures].[Sales Amount] - [Measures].[Cost of Goods Sold]'
                SELECT {[Measures].[Sales Amount], [Measures].[Gross Profit]} ON COLUMNS,
                       [Date].[Calendar Year].Members ON ROWS
                FROM [YourCube]

Creating Calculated Members in Visual Studio:

  1. Open your SSAS project in Visual Studio.
  2. Navigate to the cube you are working with.
  3. In the "Calculations" tab, click "New Calculated Member".
  4. Define the member name, parent hierarchy, and the MDX expression.

Understanding Calculation Order and SCOPE

The order in which calculations are processed can significantly impact the final results. The SCOPE statement allows you to control the context in which a calculation is applied. This is crucial for complex scenarios, especially when dealing with predefined aggregations or currency conversions.

Tip: Understanding the difference between calculations defined in MDX queries (temporary) and those defined within the cube (persistent) is essential. Cube-defined calculations are available to all users querying that cube.

Best Practices for MDX Calculations

  • Keep it Readable: Use comments and proper indentation in your MDX code.
  • Optimize Performance: Avoid unnecessarily complex calculations. Test the performance of your MDX queries.
  • Leverage Time Intelligence: Use built-in time intelligence functions where appropriate.
  • Use Named Sets: For frequently used complex sets, named sets can improve performance and code clarity.
  • Document Your Logic: Especially for complex calculations, document the business logic behind them.

Further Reading