How to calculate rolling 12 month result

Hi Expert,

Thanks for your help

I have checked some topic for this and they are suggest using Row-1,-2 to calcualte this

But my data structure is like this

Customer YearMonth GSV

A              201201       100

A              201203        50

B              201301        400

A              201209        1000

I need to have a new colum as rolling 12 month GSV

But as you could see for customer A, it is NOT always has slaes for each month

So I need to use a loop like

SELECT * from table where customer = current_row customer and yearmonth >= current month - 12 and yearmonth <current month

Customer YearMonth GSV   Rolling_12

A              201201       100    0

A              201203        50    100

B              201301        400  0

A              201209        1000 150

Please kindly let me know how to achieve this

Best regards

Alex yang