SQL & PL/SQL :: Code To Update Table In 11g

I have a table extended_values_Test, with the following structure:

CREATE TABLE extended_values_Test
(
MATRL_NUM VARCHAR2(18 BYTE),
PRI_SHIP_LOC CHAR(4 BYTE) NOT NULL,
PLANT CHAR(4 BYTE) NOT NULL,
STD_COST NUMBER,
SNAPSHOT_DATE CHAR(8 BYTE)
)

Snapshot_date is last working date of that month, and is recorded only once in the table for a matrl_num & plant combination.

Inserting records in the table:
Refer to insert_extended_values.sql
Note: This is just a sample table , having records for only two matrl_num.

after inserting records , you can see the records are for two different matrl_num's. For each matrl_num , we can have multiple values of plant and for each plant we can have different value of std_Cost(eg:matrl_num='0023173556').

Moreover for each matrl_num , I have some records in each month , based on the number of plants(with std_cost). Std_cost for a plant is updated in the table whenever there is a change in it from the previous value.In case of no change in value of std_cost over previous month's value , we have 'Null' std_cost for that matrl_num, plant & month(next month).

I want to update these 'Null' values with latest(last month's 'not null' - std_cost) std_cost for that matrl_num and plant. Such change in std_cost can occur multiple times , so we need to update the 'Null' std_cost till the time we have encountered a change in std_cost and so on.