DataModule2.ZQueryTable1.Close;
DataModule2.ZQueryTable1.SQL.Clear;
DataModule2.ZQueryTable1.SQL.Add('SELECT');
DataModule2.ZQueryTable1.SQL.Add('cod1, case when row_number() over(partition by cod1 order by cod2)=1 then cod1 else null end,cod2');
DataModule2.ZQueryTable1.SQL.Add('FROM Table1');
DataModule2.ZQueryTable1.Open;

SELECT 
IF(@prev_value=cod1,@row_num+1 and cod1=null,cod1) AS codnew,
@row_num := IF(@prev_value=cod1,@row_num+1,1) AS RowNumber,
cod1, cod2,@prev_value := cod1
FROM Table1
,(SELECT @row_num := 1) x, 
(SELECT @prev_value := Null) y   
ORDER BY cod1,cod2  DESC