Sub-queries supported in Analysis Services MDX

MDX now supports sub-queries, allowing you to change the criteria of the outer query dynamically, depending on the context returned by the sub-query. Each MDX query is executed with a context of a multi-dimensional space (sometimes referred to as a sub-cube). If a query doesn't include a sub-query, then the context is the entire cube. For example, let's suppose we wanted to reduce the previous query to only return the calculations related to the first quarter of fiscal year 2001. We can alter the query as follows:

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Note that the sub-query is included in the FROM clause, as opposed to Transact-SQL where a sub-query would typically be part of the WHERE clause. As expected, the results will be limited to the first quarter of fiscal year 2001:

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This example is simplistic. Of course we could ju

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st as easily include the DESCENDANTS function in the original query to limit the result set. However, sub-queries can come in extremely handy if you have a complicated WHERE clause that contains multiple members. This is important because, although MDX allows sets in the WHERE clause (another welcomed improvement in Analysis Services 2005), it only allows one member from each hierarchy. For example, we could modify the following query to limit the output to only the sales occurring in Australia and Canada:

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The output would look like this:

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In previous versions of Analysis Services, the only way to accomplish similar functionality would be to create a named set and use AGGREGATE function. Unfortunately, AGGREGATE function had many limitations, not the least of which was its performance.

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