Union
Returns a set that is generated by the union of two sets. By default, duplicates are deleted from the tail end of the set; however, you can optionally specify to retain duplicates.
Syntax
Union(Set_Expression1, Set_Expression2 [,...n][, ALL])
Input parameters
Set_Expression1
An MDX expression that returns a set. This must consist of tuple sets from the same hierarchy as Set_Expression2.
Set_Expression2
An MDX expression that returns a set. This must consist of tuple sets from the same hierarchy as Set_Expression1.
ALL
If specified, duplicates are retained in the joined set.
Example
Union( [DateCustom].[StandardMonth].CurrentMember.Siblings, [DateCustom].[StandardMonth].CurrentMember.Lag(1).Siblings )