To see an excellent introduction about Microsoft's multidimensional expressions MDX , follow this link:

http://msdn.microsoft.com/library/en-us/dnolap/html/intromdx.asp

Introduction to Multidimensional Expressions (MDX)

Summary: Introduces multidimensional expressions (MDX), a highly functional expression syntax for querying multidimensional data in Microsoft® SQL Server OLAP Services. Discusses the structure of OLAP Services cubes and explores the features of MDX.