Koen's nailed all the good reasons for not persisting this information. It's easy enough to calculate on the fly:

-- This running total uses a triangular join.

-- The performance of TJ's usually sucks. Depending

-- on the maximum number of rows which are

-- aggregated when month = 12, you may have to change

-- this part of the query to a running totals rCTE

-- or a Quirky Update

;WITH RunningTotals AS (

SELECT t1.cod, t1.year, t1.month, [value] = x.value

FROM Table_1 t1

CROSS APPLY (

SELECT value = SUM(value)

FROM Table_1 t2

WHERE t2.cod = t1.cod

AND t2.year = t1.year

AND t2.month <= t1.month) x

),

-- construct a matrix containing all cod/year/month values

AllCodPeriods AS (

SELECT

AllCods.cod,

AllYears.year,

AllMonths.month

FROM (SELECT month FROM RunningTotals GROUP BY month) AllMonths

CROSS JOIN (SELECT year FROM RunningTotals GROUP BY year) AllYears

CROSS JOIN (SELECT cod FROM RunningTotals GROUP BY cod) AllCods

)

SELECT

matrix.cod,

matrix.year,

matrix.month,

ty.value,

[value pp] = ly.value

FROM AllCodPeriods matrix

LEFT JOIN RunningTotals ty

ON ty.cod = matrix.cod

AND ty.year = matrix.year

AND ty.month = matrix.month

LEFT JOIN RunningTotals ly

ON ly.cod = matrix.cod

AND ly.year+1 = matrix.year

AND ly.month = matrix.month

-- here's an alternative using APPLY, which carries down

-- the running total from 2012/2 to 2012/3.

SELECT

AllCods.cod,

AllYears.year,

AllMonths.month,

ty.Value,

[value pp] = ly.value

FROM (SELECT month FROM Table_1 GROUP BY month) AllMonths

CROSS JOIN (SELECT year FROM Table_1 GROUP BY year) AllYears

CROSS JOIN (SELECT cod FROM Table_1 GROUP BY cod) AllCods

OUTER APPLY (

SELECT value = SUM(value)

FROM Table_1 t2

WHERE t2.cod = AllCods.cod

AND t2.year = AllYears.year

AND t2.month <= AllMonths.month

) ty

OUTER APPLY (

SELECT value = SUM(value)

FROM Table_1 t2

WHERE t2.cod = AllCods.cod

AND t2.year+1 = AllYears.year

AND t2.month <= AllMonths.month

) ly