MDX Syntax: A Comprehensive Guide

Multidimensional Expressions (MDX) is a query language for OLAP (Online Analytical Processing) databases, most notably Microsoft SQL Server Analysis Services.

Introduction to MDX

MDX is a powerful and flexible language that allows you to retrieve, manipulate, and analyze data stored in multidimensional cubes. It is designed to express queries against OLAP data structures, enabling complex slicing, dicing, and aggregation operations.

Core Concepts

Understanding these core concepts is crucial for writing effective MDX queries:

  • Cubes: The central data structure in Analysis Services, representing multidimensional data.
  • Dimensions: Represent categories of data, such as Time, Geography, or Products.
  • Hierarchies: Structures within dimensions that define levels of aggregation (e.g., Year > Quarter > Month).
  • Members: Individual items within a hierarchy (e.g., '2023' in the Year level).
  • Sets: An ordered collection of members or tuples.
  • Tuples: A collection of members, one from each of a specified set of hierarchies.
  • Measures: Numerical values aggregated from the fact table, such as Sales Amount or Quantity.

Basic MDX Query Structure

A typical MDX query consists of the following clauses:

  • SELECT: Specifies the axes of the query (columns and rows).
  • FROM: Specifies the cube that the query operates on.
  • WHERE: Specifies a slicer that filters the data context.

Example: Simple Sales Query

This example retrieves the total sales for each product in 2023:


SELECT
    {[Measures].[Internet Sales Amount]} ON COLUMNS,
    {[Product].[Category].Members} ON ROWS
FROM
    [Adventure Works DW2019]
WHERE
    ([Date].[Calendar Year].&[2023])

MDX Functions

MDX provides a rich set of functions for data manipulation and analysis. Some common categories include:

  • Set Functions: Manipulate sets of members (e.g., NON EMPTY, HEAD, TAIL, UNION).
  • Member Functions: Retrieve information about members (e.g., MEMBER_NAME, PARENT, CHILDREN).
  • Numeric Functions: Perform calculations and aggregations (e.g., SUM, AVG, COUNT).
  • String Functions: Manipulate string values.

Example: Using a Set Function

This query retrieves the top 5 products by sales amount:


SELECT
    NON EMPTY
    TOP 5
    {[Measures].[Internet Sales Amount]} ON COLUMNS,
    {[Product].[Product Name].Members} ON ROWS
FROM
    [Adventure Works DW2019]
WHERE
    ([Date].[Calendar Year].&[2023])

Working with Hierarchies and Levels

MDX allows you to navigate through hierarchies to access data at different granularities.

  • Current Member: The currently selected member within a hierarchy.
  • Ancestor/Descendant: Functions to traverse up or down a hierarchy.

Example: Accessing Parent Member

This query shows sales for each product within its category:


SELECT
    {[Measures].[Internet Sales Amount]} ON COLUMNS,
    {[Product].[Product Name].Members * [Product].[Category].Members} ON ROWS
FROM
    [Adventure Works DW2019]
WHERE
    ([Date].[Calendar Year].&[2023])

Advanced MDX Features

  • Calculated Members: Define new members on the fly for custom calculations.
  • Named Sets: Define reusable sets for complex queries.
  • Subcubes: Query subsets of a cube.
  • Session Variables: Store temporary values within a session.

Further Learning

To deepen your understanding of MDX, explore the following resources: