Join Compatibility

  • 项目

The FROM clause is used to specify the name of the source cube. Some providers may support joining cubes that share common dimensions. These providers can specify a comma-delimited list of cubes to be joined in the FROM clause. For more information about defining cubes, see Establishing Cube Context.

Note

To perform a join, it is not necessary to specify the relationship between cubes, because joining is always by common dimensions, meaning dimensions that have the same name. Two cubes are said to have common dimensions if and only if the dimensions have the same DIMENSION_UNIQUE_NAME. For more information, see "Multiple Hypercubes vs. True Multicubes" in the section Hypercubes and Multicubes.

The semantics of joining by common dimensions, called a natural join, can be expressed as follows:

  1. Let the notation X(A, B, C) indicate a cube X that is composed of dimensions A, B, and C.

  2. Let X(A, B, C) and Y(C, D, E) be two cubes with the common dimension C.

  3. If X.C and Y.C do not have an identical set of members, then X and Y cannot be joined.

  4. Assuming X.C and Y.C have an identical set of members, the natural join of X and Y results in a cube XY(A, B, C, D, E) such that the dataset

    SELECT XY.A ON COLUMNS, XY.B ON ROWS, XY.C ON PAGES
WHERE (D.[ALL], E.[ALL])

    is the same as the cube X(A, B, C). And the dataset

    SELECT XY.C ON COLUMNS, XY.D ON ROWS, XY.E ON PAGES
WHERE (A.[ALL], B.[ALL])

    is the same as the cube Y(C, D, E).

The natural join operation associates and commutes (except for the dimension order).

The rules explained in the preceding paragraphs define semantics for cubes that have three dimensions each and one dimension common to both cubes; however, the generalization to other cases is straightforward.

The method of joining cubes described here is similar to a natural join in a relational database system. Vendors can extend the language to support other types of joins.