Function field_ergtbl_exists(con As Connection, sernr As String) As Boolean
 
Dim rs      As Recordset
 
field_ergtbl_exists = False
 
Set rs = New Recordset 
With rs
    Set .ActiveConnection = con
        .CursorLocation = adUseClient
        .CursorType = adOpenKeyset
        .LockType = adLockOptimistic
        .Source = "SELECT Sernr FROM Ergebnis_tbl;"
        .Open Options:=adCmdText            
End With

For i = 0 To rs.RecordCount - 1
        If rs.Fields(i).Value = sernr Then            
            field_ergtbl_exists = True          
            Exit Function
        End If
Next i