Set r1=DB.OpenRecordset("SELECT a FROM Index group by a")
If r1.RecordCount > 0 Then
r1.MoveLast
r1.MoveFirst
Do Until r1.EOF
Set r2=DB.OpenRecordset("SELECT max(b) As [Max] FROM Index where a=" & r1("a"))
DB.Execute "UPDATE Index SET c=1 where a=" & r1("a") & " and b=" & r2("Max")
r1.MoveNext
Loop
End If