MDX 学习
Members, Tuples, and Sets
Before proceeding on the creation of a Multidimensional Expressions (MDX) query, you should understand the definitions of members, tuples and sets, as well as the MDX syntax used to construct and refer to these elements.
Members
A member is an item in a dimension representing one or more occurrences of data. Think of a member in a dimension as one or more records in the underlying database whose value in this column falls under this category. A member is the lowest level of reference when describing cell data in a cube.
For example, the following diagram is shaded to represent the Time.[2nd half].[3rd quarter] member.
The bracket characters, [ and ], are used if the name of a member has a space or a number in it. Although the Time dimension is one word, bracket characters can also be used around it as well; the member shown in the previous diagram could also be represented as:
[Time].[2nd half].[4th quarter]
The right bracket (]) can be used as an escape character in MDX if the member name or member key contains a right bracket, as shown in the following example:
[Premier [150]] 98]
Member Names and Member Keys
A member can be referenced by either its member name or by its member key. The previous example referenced the member by its member name, 4th quarter, in the Time dimension. However, the member name can be duplicated in the case of dimensions with nonunique member names, or it can be changed in the case of changing dimensions.
An alternate method to reference members is by referencing the member key. The member key is used by the dimension to specifically identify a given member. The ampersand (&) character is used in MDX to differentiate a member key from a member name, as shown in the following example:
[Time].[2nd half].&[Q4]
In this case, the member key of the 4th quarter member, Q4, is used. Referencing the member key ensures proper member identification in changing dimensions and in dimensions with non-unique member names.
The ampersand character can be used to indicate a member key reference in any MDX expression.
Calculated Members
Members can also be created, as part of an MDX query, to return data based on evaluated expressions instead of stored data in a cube to be queried. These members are called calculated members, and they provide a great deal of the power and flexibility of MDX. The WITH keyword is used in an MDX query to define a calculated member. For example, if you want to provide a forecast estimate all of the packages by adding 10% of the existing value of the Packages measure, you can simply create a calculated member that provides the information and use it just like any other member in the cube, as demonstrated in the following example.
WITH MEMBER [Measures].[PackagesForecast] AS
'[Measures].[Packages] * 1.1'
For more information, see Calculated Members.
Member Functions
MDX supplies a number of functions for retrieving members from other MDX entities, such as dimensions and levels, so that explicit references to a member are not always necessary. For example, the FirstChild function allows the retrieval of all the members from a given dimension or level; to get the first child member of the Time dimension, you can explicitly state it, as demonstrated in the following example:
Time.[1st half]
You can also use the FirstChild function to return the same member, demonstrated in the next example.
Time.FirstChild
For more information about MDX member functions, see MDX Function List.
Tuples
A tuple is used to define a slice of data from a cube; it is composed of an ordered collection of one member from one or more dimensions. A tuple is used to identify specific sections of multidimensional data from a cube; a tuple composed of one member from each dimension in a cube completely describes a cell value. Put another way, a tuple is a vector of members; think of a tuple as one or more records in the underlying database whose value in these columns falls under these categories. A series of diagrams presents different types of tuples.
The shaded area of the cube represents the (Time.[2nd half]) tuple. Note that this tuple encompasses half of the cube, because it does not rule out any information in the Source or Route dimensions.
The following diagram is shaded to represent the (Time.[2nd half], Route.nonground.air) tuple.