CountDistinct Function (Reporting Services)
Returns a count of all distinct non-null values specified by the expression, evaluated in the context of the given scope.
The following code example shows an expression that calculates the number of unique non-null values of Size for the default scope and for a parent group scope. The expression is added to a cell in a row that belongs to the child group GroupbySubcategory. The parent group is GroupbyCategory. The expression displays the results for GroupbySubcategory (the default scope) and then for GroupbyCategory (the parent group scope).
Expressions should not contain actual carriage returns and line breaks; these are included in the example code to support documentation renderers. If you copy the following example, remove carriage returns from each line.
="Distinct count (Subcategory): " & CountDistinct(Fields!Size.Value) & "Distinct count (Category): " & CountDistinct(Fields!Size.Value,"GroupbyCategory")