MDX Syntax Overview - SQL Server Analysis Services Multidimensional Modeling

Introduction to MDX

Multidimensional Expressions (MDX) is a query language designed for querying and manipulating multidimensional data stored in OLAP (Online Analytical Processing) cubes. Developed by Microsoft, MDX is the standard query language for SQL Server Analysis Services (SSAS) and is widely used in business intelligence solutions.

MDX queries allow you to:

Retrieve data from multidimensional structures (cubes, dimensions, hierarchies, members).
Perform complex calculations, aggregations, and analysis.
Format and present data in various ways.
Define calculated members and sets.

Core Concepts

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

Cubes: The central data structure in SSAS, containing measures and dimensions.
Dimensions: Represent the perspectives from which data can be analyzed (e.g., Time, Geography, Product).
Hierarchies: Organize members within a dimension into levels (e.g., Year -> Quarter -> Month in a Time dimension).
Members: Individual items within a hierarchy (e.g., "2023", "North America", "Bikes").
Measures: Numerical values that can be aggregated (e.g., Sales Amount, Quantity Sold).
Tuples: An ordered set of members, one from each of one or more dimensions. Tuples define a specific point in the cube.
Sets: An unordered collection of tuples. Sets are fundamental for retrieving and manipulating slices of data.

Basic MDX Query Structure

A typical MDX query has the following structure:


SELECT
    {[Measures].[Your Measure]} ON COLUMNS,
    {[Your Dimension].[Your Hierarchy].[Your Member]} ON ROWS
FROM
    [Your Cube Name]
WHERE
    ([Your Dimension].[Your Hierarchy].[Your Slicer Member])

Key Clauses:

SELECT: Specifies the axes (COLUMNS, ROWS, SLICERS) and the data to retrieve.
FROM: Identifies the cube to query.
WHERE: Acts as a slicer, filtering the query results based on specific member selections.

Working with Sets

Sets are central to MDX. They are often used on the SELECT statement axes to define the structure of the returned data. Common set functions include:

{...}: Defines a set of members or tuples.
{([Dimension].[Hierarchy].[Member1]), ([Dimension].[Hierarchy].[Member2])}: A set of tuples.
Members([Dimension].[Hierarchy]): Returns all members of a specified hierarchy.
Children([Member]): Returns the children of a given member.
Descendants([Member], [Level]): Returns all descendants of a member up to a specified level.
Crossjoin({Set1}, {Set2}): Creates a new set by combining all possible tuples from the input sets.

Calculated Members

MDX allows you to define temporary calculated members that are not stored in the cube but are computed on the fly. This is done using the WITH keyword.


WITH MEMBER [Measures].[Average Sales] AS
    Sum(
        {[Measures].[Sales Amount]},
        [Measures].[Quantity Sold]
    ) / Sum(
        {[Measures].[Quantity Sold]},
        [Measures].[Quantity Sold]
    )
SELECT
    {[Measures].[Average Sales]} ON COLUMNS
FROM
    [Your Cube Name]

Common MDX Functions

MDX provides a rich set of functions for data manipulation and calculation:

Aggregation Functions: SUM(), AVG(), COUNT(), MIN(), MAX()
Set Functions: NONEMPTY(), ORDER(), UNION()
Member Functions: MEMBER_NAME(), MEMBER_CAPTION(), PARENT()
Numeric Functions: FORMAT_STRING(), ROUND()

Conclusion

This overview provides a foundational understanding of MDX syntax. Mastering MDX requires practice with its various functions, syntax structures, and the ability to think multidimensionally. For deeper dives, refer to the official Microsoft documentation.