MDX Guide for Multidimensional Models

Master the Multidimensional Expressions language for Analysis Services.

Introduction to MDX

Multidimensional Expressions (MDX) is a query language designed for online analytical processing (OLAP) and business intelligence applications. It allows you to retrieve and manipulate data from multidimensional data sources, such as Microsoft SQL Server Analysis Services (SSAS) cubes.

MDX provides a powerful way to slice, dice, and aggregate data, enabling users to explore complex business data from various perspectives. This guide will walk you through the fundamental concepts and advanced features of MDX.

Getting Started with MDX

To start writing MDX queries, you typically need access to an Analysis Services instance and a multidimensional cube. Tools like SQL Server Management Studio (SSMS) or dedicated BI development environments provide interfaces for writing and executing MDX queries.

A basic MDX query structure involves selecting measures and dimensions, and specifying the context of the query.

Basic MDX Syntax

MDX syntax is case-insensitive, but it's good practice to maintain a consistent casing for readability. Key components include:

  • Keywords: Reserved words like SELECT, FROM, ON, WHERE, WITH.
  • Identifiers: Names of cubes, dimensions, hierarchies, levels, members, measures, and calculated members. These are often enclosed in square brackets ([]) if they contain spaces or special characters.
  • Sets: Collections of members, tuples, or other sets.
  • Tuples: Ordered combinations of members from different dimensions.

Querying Data with MDX

The core of MDX querying revolves around the SELECT statement, which retrieves data from a cube.

The SELECT Statement

The SELECT statement specifies the data you want to retrieve. It defines axes (like rows and columns) and the slicer (context).

The FROM Clause

The FROM clause specifies the cube from which data is being retrieved.

FROM [Your Cube Name]

The ON AXIS Clause

The ON AXIS clause defines what appears on each axis of the result set. Common axes are COLUMNS and ROWS.

Example:

SELECT
    {[Measures].[Sales Amount]} ON COLUMNS,
    {[Date].[Calendar Year].Members} ON ROWS
FROM [Your Cube Name]

The WHERE Clause (Slicer)

The WHERE clause, often referred to as the slicer, filters the entire query by specifying a context. It's typically used to select a single member from a dimension.

Example:

SELECT
    {[Measures].[Sales Amount]} ON COLUMNS,
    {[Date].[Calendar Year].Members} ON ROWS
FROM [Your Cube Name]
WHERE ([Geography].[Country].&[United States])

MDX Functions

MDX offers a rich library of functions for data manipulation, calculation, and analysis. Here are a few categories:

Numeric Functions

Used for mathematical calculations and aggregations.

  • Sum(Set, Numeric Expression): Returns the sum of a numeric expression evaluated over a set.
  • Avg(Set, Numeric Expression): Returns the average of a numeric expression.
  • Count(Set): Returns the number of members in a set.

String Functions

Used for manipulating text data.

  • Left(String, Integer): Returns the left portion of a string.
  • Right(String, Integer): Returns the right portion of a string.
  • Upper(String): Converts a string to uppercase.

Set Functions

Used to manipulate collections of members or tuples.

  • Members(Hierarchy): Returns all members of a hierarchy.
  • Children(Member): Returns the children of a member.
  • Filter(Set, Condition): Returns a subset of a set based on a condition.
  • TopCount(Set, Count, Numeric Expression): Returns the top N members of a set based on a numeric expression.

Member Functions

Used to retrieve information about members.

  • CurrentMember(Dimension): Returns the current member in the context of an iterator.
  • Parent(Member): Returns the parent member of a given member.
  • Level(Member): Returns the level of a member.

Advanced MDX Topics

Beyond basic querying, MDX supports more complex analytical scenarios.

Calculated Members

Calculated members allow you to define new measures or members within your cube definition or on-the-fly in your query. They are defined using the WITH clause.

WITH MEMBER [Measures].[Profit Margin] AS
    ([Measures].[Sales Amount] - [Measures].[Cost Amount]) / [Measures].[Sales Amount]

SELECT
    {[Measures].[Sales Amount], [Measures].[Profit Margin]} ON COLUMNS,
    {[Date].[Calendar Year].Members} ON ROWS
FROM [Your Cube Name]

Sub-Cubes

Sub-cubes allow you to define a smaller cube within a larger one, often used for performance optimization or specific analysis. The { ... } ON CURSOR syntax is used.

MDX Scripting

MDX scripts are used within cube definitions to initialize session variables, set member properties, and perform complex calculations that affect the entire cube or session, often executed before queries are processed.

Best Practices for MDX

Writing efficient and maintainable MDX queries is crucial for performance.

  • Understand your cube structure: Know your dimensions, hierarchies, levels, and measures.
  • Use specific members: Avoid `Members` or `All` members unless necessary. Drill down to specific members.
  • Leverage the slicer (WHERE clause): Use it to filter the query context effectively.
  • Optimize set functions: Be mindful of the performance implications of functions like Generate or recursive functions.
  • Use calculated members judiciously: Define complex calculations once in the cube rather than repeatedly in queries if possible.
  • Test and Profile: Use SSMS execution plans and performance counters to identify bottlenecks.
  • Readability: Use consistent formatting, comments, and meaningful names.