EXISTING keyword

As the documentation states the EXISTING keyword forces a specified set to be evaluated within the current context instead of being evaluated within the context of the cube that contains the members of the set. Mosha Pasumasky declares "that Exists function and EXISTING operator are generalization of hierarchy related functions such as Descendants. Indeed, the process of building an hierarchy is nothing more then applying Exists between attributes which build individual levels of that hierarchy." So EXISTING shows only elements that are actually relevant in the given context.

If we are looking for the number of cities in each country in the Adventure Works Cube and use the following query:

WITH
  MEMBER [CITY_COUNT] AS COUNT([GEOGRAPHY].[CITY].[CITY].MEMBERS)
SELECT {[CITY_COUNT]} ON 0
       ,[GEOGRAPHY].[GEOGRAPHY].[COUNTRY].MEMBERS ON 1
  FROM [ADVENTURE WORKS]  

we do not get the intended result:

                CITY_COUNT
Australia              587
Canada                 587
France                 587
Germany                587
United Kingdom         587
United States          587

That's the total count of cities for every country - apparently the evaluation took place in the context of the cube and not in the current context. With EXISTING we get what we want:

WITH
  MEMBER [CITY_COUNT] AS COUNT(EXISTING([GEOGRAPHY].[CITY].[CITY].MEMBERS))
SELECT {[CITY_COUNT]} ON 0
       ,[GEOGRAPHY].[GEOGRAPHY].[COUNTRY].MEMBERS ON 1
  FROM [ADVENTURE WORKS]  

provides the result:

                CITY_COUNT
Australia               40
Canada                  50
France                  36
Germany                 51
United Kingdom          35
United States          375

As already mentioned here EXISTING can also be used to solve some problems with multiselects.

In his Blog Chris Webb wrote about the performance of EXISTING in combination with NONEMPTY and stated "if you are using NonEmpty and Existing together, do the NonEmpty first and then the Existing" - but admitted that he has no sound explanation for the observed effect.