Key Concepts in MDX
The purpose of Multidimensional Expressions (MDX) is to make accessing data from multiple dimensions easier and more intuitive.
Dimensions, Levels, Members, and Measures
Most languages used for data definition and manipulation, such as SQL, are designed to retrieve data in two dimensions: a column dimension and a row dimension. The following diagram illustrates a traditional relational database, used to store order information.
Each table represents two-dimensional data. At the intersection of each row and column is a single element of data, called a field. The specific columns to be viewed in an SQL query are specified with a SELECT statement, and the rows to be retrieved are limited by a WHERE clause.
Multidimensional data, on the other hand, can be represented by structures with more than two dimensions. These structures, called cubes, have multiple dimensions. At the intersection of dimensions in a cube, there may be more than one element of data, called a measure. The following diagram illustrates a cube that employs three dimensions, Route, Service and Time; and two measures, Packages and Last. Each dimension is broken down into different levels, each of which is broken down further into members. For example, the Source dimension supplies the Eastern Hemisphere level, which is broken down into four members, Africa, Asia, Australia, and Europe.
As you can see, the querying of even simple data out of a multidimensional data source can be a complex task. A cube can have more than three dimensions, for example, or it may only have one dimension.
The concepts of cubes, dimensions, levels, members, and measures are important to the understanding of MDX syntax. Further reading on these architectural topics is recommended if you are new to online analytical processing (OLAP) databases.
Cells, Tuples, and Sets
As SQL returns a subset of two-dimensional data from tables, MDX returns a subset of multidimensional data from cubes.
The cube diagram illustrates that the intersection of multidimensional members creates cells from which you can obtain data. To identify and extract such data, whether it be a single cell or a block of cells, MDX uses a reference system called tuples. Tuples list dimensions and members to identify individual cells as well as larger sections of cells in the cube; because each cell is an intersection of all the dimensions of the cube, tuples can uniquely identify every cell in the cube. For the purposes of reference, measures in a cube are treated as a private dimension, named Measures, in the cube itself. For example, in the preceding diagram, the following tuple identifies a cell in which the value is 400:
(Source.[Eastern Hemisphere].Africa, Time.[2nd half].[4th quarter], Route.Air, Measures.Packages)
The tuple uniquely identifies a section in the cube; it does not have to refer to a specific cell, nor does it have to encompass all of the dimensions in a cube. The following examples are all tuples of the cube diagram:
(Source.[Eastern Hemisphere])
(Time.[2nd half], Source.[Western Hemisphere])
These tuples provide sections of the cube, called slices, that encompass more than one cell.
An ordered collection of tuples is referred to as a set. In an MDX query, axis and slicer dimensions are composed of such sets of tuples. The following example is a description of a set of tuples in the cube in the diagram:
{ (Time.[1st half].[1st quarter]), Time.[2nd half].[3rd quarter]) }
In addition, it is possible to create a named set. A named set is a set with an alias, used to make your MDX query easier to understand and, if it is particularly complex, easier to process.
Axis and Slicer Dimensions
In SQL, it is usually necessary to restrict the amount of data returned from a query on a table. For example, you may want to see only two fields of a table with forty fields, and you want to see them only if a third field meets a specific criteria. You can accomplish this by specifying columns in the SELECT statement, using a WHERE statement to restrict the rows that are returned based on specific criteria.
In MDX, those concepts also apply. A SELECT statement is used to select the dimensions and members to be returned, referred to as axis dimensions. The WHERE statement is used to restrict the returned data to specific dimension and member criteria, referred to as a slicer dimension. An axis dimension is expected to return data for multiple members, while a slicer dimension is expected to return data for a single member.
The terms "axis dimension" and "slicer dimension" are used to differentiate the dimensions of the cells in the source cube of the query, indicated in the FROM clause, from the dimensions of the cells in the result cube, which can be composed of multiple cube dimensions.
Calculated Members
Calculated members are members that are based not on data, but on evaluated expressions in MDX. They are returned in the same fashion as a normal member. MDX supplies a robust set of functions that can be used to create calculated members, giving you extensive flexibility in the manipulation of multidimensional data.
User-Defined Functions
MDX provides extensibility in the form of user-defined functions using any programming language that can support Component Object Model (COM) interfaces. You create and register your own functions that operate on multidimensional data as well as accept arguments and return values in the MDX syntax. You can call user-defined functions from within Calculated Member Builder, data definition language (DDL) statements that support MDX, and MDX queries.
PivotTable Service
In Microsoft® SQL Server™ 2000 Analysis Services, MDX data definition and manipulation services are provided through PivotTable® Service. PivotTable Service also provides stand-alone OLE DB provider capabilities for multidimensional queries when not connected to an Analysis server. PivotTable Service is used for the definition and manipulation of local cubes, which can be used to locally store data in a multidimensional format.