CountDistinct - How About SumDistinct - Or Just AggregateDistinct?

Making a report in Microsoft SQL Server 2005 Reporting Services today I needed to sum up several levels in a table. However, the numbers where available only at an intermediate aggregated level, so I needed to sum up only the distinct sublevel-items. Got that? Maybe it's easier if I show an example:

Let's concoct an example here. I'm extracting data in the following groups: Firm, ProductType, Product, Account, Item. Say I'm summing up one measure, BalanceAmount, on products, but my dataset returns other measures on the lowest level, Item.

Here's an example DataSet

Firm	ProductType	Product	Account	Item	BalanceAmount	ItemMeasure
Acme	Explosive	Nitroglyserin	C-001	Bottle	$200	$2.14
Acme	Explosive	Nitroglyserin	C-001	Mug	$200	$4.87
Acme	Explosive	TNT	C-001	Stick	$200	$0.79
Acme	Explosive	Plastique	C-001	Wad	$200	$7.64
Acme	Prop	Fake rock	C-001	Crate	$200	$4.55
Acme	Explosive	TNT	C-124	Stick	$89	$0.79
Acme	Explosive	TNT	C-124	Half-stick	$89	$0.40
Acme	Explosive	Nitroglyserin	C-089	Stick	$40	$0.79

In the report I need to show this as a drill-down table, with the measures down on the lowest level (Item) summed up on every level, and the aggregated measure (BalanceAmount) summed up on every level where it makes sense.

The problem is that if you use the common SUM function the report would sum up all BalanceAmounts on the Account level resulting in a BalanceAmount of $1000 for Account C-001, not $200 which is the correct value (that would be $200, Einstein).

What you really need is something like CountDistinct, but just for Sum. In fact, this is a specific case of a more general problem where you need to conduct some aggregation on some scope. How do I do something like that?

It's not really hard. What I need is some way to make just the first instance of a value in whatever scope needed be the counting one. To get this I enter the
following into the Code tab of you Report properties:

    Dim scopes As System.Collections.Hashtable
  Function getDistinct(ByVal checkMe As Object, ByVal scope As String) As Boolean
      Dim firstTimeSeen As Boolean

      If (scopes Is Nothing) Then
          scopes = New System.Collections.Hashtable
      End If

      If (Not scopes.Contains(scope)) Then
          scopes.Add(scope, New System.Collections.ArrayList)
      End If

      If (Not CType(scopes(scope), System.Collections.ArrayList).Contains(checkMe)) Then
          firstTimeSeen = True
          CType(scopes(scope), System.Collections.ArrayList).Add(checkMe)
      Else
          firstTimeSeen = False
      End If

      Return firstTimeSeen
  End Function

The usage pattern to get SumDistinct in this case is:

=SUM(IIF(Code.getDistinct(Fields!account.Value, "scopeId"), Fields!BalanceAmount.Value, 0))

The "scopeId" string identifies the current scope, so it will change from group to group. E.g. in the Firm group you'd enter "firm", in the ProductType group you'd enter "prodType", etc.

So what am I doing here? We're remembering the values of the checkMe parameter in the context of the scope, and returning True in only the first instance. Thus we can just sum the first of every checkMe value. You could use this to do your own CountDistinct by entering the following expression (but this would be a bit roundabout for that use-case):

=COUNT(IIF(Code.getDistinct(Fields!account.Value, "scopeId"), Fields!BalanceAmount.Value, 0))

I hope you get some use out of this. This kind of code is easy to crank out, but even easier to just copy-paste.