Last_Value returning the current value

I am not able to get the last value, rather it is just returning the same value with my code below in snowflake – does anyone have any idea? Is there something glaring wrong?

select MNTH, 
       sum_cust, 
       last_value(sum_cust) over (partition by MNTH order by sum_cust desc ROWS BETWEEN UNBOUNDED PRECEDING AND UNBOUNDED FOLLOWING) as sum_cust_last
from block_2;

Answer

I think what you actually want is to LAG the value from the previous MNTH:

SELECT MNTH, 
       sum_cust, 
       LAG(sum_cust) OVER (ORDER BY MNTH) AS sum_cust_last
FROM block_2;