MDX Syntax and Semantics

This document provides a comprehensive overview of the Multidimensional Expressions (MDX) syntax and semantics used in SQL Server Analysis Services. MDX is a query language that allows you to retrieve data from OLAP cubes and perform complex analytical operations.

Core Concepts

Statements and Commands

MDX queries are structured as statements. Common statements include:

• SELECT: Used to retrieve data from a cube.
• WITH: Used to define temporary named sets or calculated members.
• UPDATE CUBE: Used to modify cube data (less common for read operations).

Objects

MDX operates on various objects within an Analysis Services database:

• Cubes: The primary data structure.
• Dimensions: Hierarchical structures within a cube (e.g., Time, Geography).
• Hierarchies: Levels within a dimension.
• Members: Specific items within a hierarchy (e.g., '2023' in the Time dimension).
• Measures: Aggregated values (e.g., Sales Amount, Quantity).
• Sets: Collections of members.
• Tuples: Ordered combinations of members.

Basic MDX Syntax

The SELECT Statement

The fundamental MDX query uses the SELECT statement. It defines which data to retrieve and how to slice and dice it.

Syntax Example

SELECT
  {[Measures].[Internet Sales Amount]} ON COLUMNS,
  {[Date].[Calendar Year].Members} ON ROWS
FROM
  [Adventure Works]
WHERE
  ([Product].[Category].&[1], [Geography].[Country].&[USA])
;
                

Axes

The ON COLUMNS and ON ROWS clauses define the axes of the result set. You can define up to 128 axes, but typically 2 are used (columns and rows).

Sets and Tuples

Defining Sets

Sets are unordered collections of members. They are often used on axes.

-- Get all members of the Calendar Year hierarchy
{[Date].[Calendar Year].Members}

-- Get specific members
{[Date].[Calendar Year].&[2022], [Date].[Calendar Year].&[2023]}
                

Defining Tuples

Tuples are ordered collections of members. They are used to specify slicers or to define points on axes.

-- A tuple representing a specific point
([Product].[Category].&[1], [Geography].[Country].&[USA])
                

Common Functions and Operators

Set Functions

• .Members: Returns all members of a level or hierarchy.
• .Children: Returns direct children of a member.
• .Parent: Returns the parent of a member.
• TopCount(), BottomCount(): Returns a specified number of top/bottom members based on a numeric expression.
• Filter(): Filters a set based on a condition.

Numeric Functions

• Sum(), Avg(), Count(): Aggregate functions.
• IIF(): Conditional logic.

Operators

• +, -, *, /: Arithmetic operators.
• =, <>, <, >, <=, >=: Comparison operators.

Calculated Members and Named Sets

Calculated Members

Calculated members are dynamic members defined within a query or cube structure.

WITH MEMBER MEASURES.[Total Sales Plus Tax] AS
  [Measures].[Internet Sales Amount] * 1.05
SELECT
  {[Measures].[Internet Sales Amount], [Measures].[Total Sales Plus Tax]} ON COLUMNS
FROM
  [Adventure Works]
;
                

Named Sets

Named sets are pre-defined sets that can be reused in queries.

WITH SET [Top 5 Products] AS
  TopCount([Product].[Product Name].Members, 5, [Measures].[Internet Sales Amount])
SELECT
  [Top 5 Products] ON ROWS
FROM
  [Adventure Works]
;
                

Semantics and Best Practices

• Case Insensitivity: MDX keywords and function names are generally case-insensitive, but object names (like cube names, dimension names) might be case-sensitive depending on server configuration.
• Member Uniqueness: Ensure members are uniquely identified using the dot notation (.) or the member key syntax (.& for keys).
• Performance Considerations: Avoid using .AllMembers on very large dimensions without proper filtering. Prefer using specific levels or filtered sets. Optimize slicers in the WHERE clause.
• Readability: Use consistent formatting, comments, and meaningful names for calculated members and named sets.