Since the table already contains the previous year's YTD totals, would a simple LEFT JOIN suffice to get the prior year's YTD total?

SELECT

t1curr.cod, t1curr.[year], t1curr.[month], t1curr.value, t1prev.value

FROM dbo.Table_1 t1curr

LEFT OUTER JOIN dbo.Table_1 t1prev ON

t1prev.cod = t1curr.cod AND

t1prev.[year] = t1curr.[year] - 1 AND

t1prev.[month] = t1curr.[month]