Calculating grand totals from group totals in Reporting Services

I have some data grouped in a table by a certain criteria, and for each group it is computed an average —well, the real case is a bit more tricky— of the values from each of the detail rows that belong to that group. This average is shown in each group footer rows. Let's see this simple example:

What I want now is to show a grand total on the table footer. The grand total should be computed by adding each group's average (for instance, in this example the grand total should be 20 + 15 = 35). However, I can't nest aggregate functions. How can I do?

Reporting Services (2005, maybe 2008, too) don't support aggregates of aggregates directly.

Use a custom report assembly, code references and named objects (Report Properties, References) that allow you to aggregate the values yourself.

Your code could look like this:

Public Sub New()

    m_valueTable = New DataTable(tableName:="DoubleValueList")

    'Type reference to System.Double
    Dim doubleType = Type.GetType(typeName:="System.Double")

    ' Add a single Double column to hold values
    m_valueTable.Columns.Add(columnName:="Value", type:=doubleType)

    ' Add aggregation column
    m_sumColumn = m_valueTable.Columns.Add(columnName:="Sum", type:=doubleType, expression:="Sum(Value)")
End Sub
Public Function Aggregate(ByVal value As Double) As Double

    ' Appends a row using a 1-element object array. 
    ' If there will be more than 1 column, more values need to be supplied respectively.
    m_valueTable.Rows.Add(value)

    Aggregate = value
End Function
Public ReadOnly Property Sum() As Double
    Get

        If 0 = m_valueTable.Rows.Count Then
            Sum = 0
        Else
            Sum = CDbl(m_valueTable.Rows(0)(m_sumColumn))
        End If
    End Get
End Property

Name you reference for example DoubleAggregator. Then replace the group expressions with "Code.DoubleAggregator.Aggregate(Avg(Fields!Column2.Value))" and the expression for Total with "Code.DoubleAggregator.Sum()".