Use Decode and Substr

Using Decode and substr together:- 

Decode wil replace the IF-THEN-ELSE

e.g

SELECT full_name,

decode(Current_employee_flag, 'Y', 'EMP',

 'N', 'Not Emp')result

FROM per_all_people_f;

Substr will filter values to the length you specify

Eg

SELECT SUBSTR('Take the first four characters', 1, 4) FIRST_FOUR
FROM dual;

To use both:

SUM(DECODE(Substr(piv.uom, 1, 1), 'M', prrv.result_value, NULL))  amount,

Will result value will grouped for numeric values.