Sql server difference instead of sum -

i have table 5 columns :

pk_column, fk_meterid, date, index, value 

and have query working fine :

select fk_meterid,        sum(value) 'total',        @p_month,        @p_year valueshistory month(date) = @p_month       , year(date)  = @p_year group  fk_meterid 

however instead of summing values on month, do

a difference of indexes between last index of month , first index of

month. (it should return same result)

how can make query using difference ?

thanks

sounds if looking aggregate functions.

with cte (     select rn = row_number() on (                   partition fk_meterid                   order date, pk_column),            total  = sum(value) on (partition fk_meterid),            minval = min(value)  on (partition fk_meterid),            maxval = max(value)  on (partition fk_meterid),            pk_column, fk_meterid, [date], [index], value     valueshistory     month(date) = @p_month       , year(date)  = @p_year ) select difference = maxval - minval, cte.* cte