Option Compare Database
Option Explicit

Private mrstClone As ADODB.Recordset

Private Sub Form_Load()
    Dim cnn As ADODB.Connection
    Set cnn = New ADODB.Connection
    cnn.Open "трампампам"
    
    Dim rst As ADODB.Recordset
    Set rst = New ADODB.Recordset
    rst.CursorLocation = adUseClient
    rst.Open "Select t.id, t.a From t", cnn, adOpenKeyset, adLockReadOnly
    
    Set Me.Recordset = rst
    
    Set mrstClone = New ADODB.Recordset
    mrstClone.CursorLocation = adUseClient
    mrstClone.Open "Select t.id, t.a From t", cnn, adOpenKeyset, adLockReadOnly
End Sub

Private Sub id_MouseUp(Button As Integer, Shift As Integer, X As Single, Y As Single)
    CalcSum
End Sub

Private Sub a_MouseUp(Button As Integer, Shift As Integer, X As Single, Y As Single)
    CalcSum
End Sub

Private Sub CalcSum()
    With mrstClone
        If Me.FilterOn Then
            .Filter = Replace(Me.Filter, "Форма1.", "")
        Else
            .Filter = ""
        End If
    
        If Me.OrderByOn Then
            .Sort = Replace(Me.OrderBy, "Форма1.", "")
        Else
            .Sort = ""
        End If
        
        Dim s As Double
        If Not (.EOF And .BOF) Then
            .MoveFirst
            .Move Me.SelTop -  1 
            
            Dim i As Long
            For i =  1  To Me.Form.SelHeight
                s = s + Nz(.fields("a"),  0 )
                .MoveNext
            Next i
        End If
    End With
    Me.Parent!txtSum = s
End Sub