ORACLE PLSQL Function/Trigger - sql

Question

I'm trying to create a FUNCTION/TRIGGER that can update the status_comunal_waste in table supply without the need of writing a query.
EDIT:
TRAFFIC (date),(input_quant),(output_quant),(status)
SUPPLY (input_quant), (output_quant),(status_comunal_waste)
After I import the DATE, INPUT, OUTPUT, STATUS data into the table TRAFFIC,
with the trigger AIR the data is also imported into the table SUPPLY, with the execption of columns USER_CHANGES, DATE_CHANGES, USER_DELETE, DATE_DELETE as to mimic the changes the "user" has made if the "user" decides to change or add the data.
So far I've tried something like this
create or replace TRIGGER supply_after_insert
after INSERT
ON traffic
FOR EACH ROW
DECLARE
Status_comunal_waste VARCHAR2(20);
BEGIN
SELECT SUM(input_quant) - SUM(output_quant)
into Status_comunal_waste
FROM traffic
WHERE (key_num_code =200101);
INSERT INTO supply
(Status_comunal_waste) VALUES (Status_comunal_waste);
END;
EDIT:
Meanwhile I switched from creating a trigger to function, and this is written so far:
create or replace
FUNCTION stat_comunal_waste (key_num_code number)
return number
is
status_comunal_waste NUMBER;
BEGIN
if (key_num_code = 200101) THEN
SELECT SUM (input_quant) - SUM (output_quant)
into status_comunal_waste
FROM traffic
WHERE key_num_code =200101;
end if;
return status_comunal_waste;
END;
The function compiled with no errors but now I get errors with the PACKAGE
CREATE OR REPLACE PACKAGE Paket
IS
function stat_comunal_waste (key_num_code number)
return status_comunal_waste;
end Paket;
/
Error(3,1): PL/SQL: Declaration ignored
Error(4,8): PLS-00488: 'status_comunal_waste' must be a type

Answer 1

This query might be wrong.
SELECT SUM(input_quant) - SUM(output_quant) into Status_comunal_waste
FROM traffic WHERE key_num_code =200101);
Make sure you are getting the correct values. Try to replace the above query with code below and give it a try
select SUM(input_quant) into input_q FROM traffic WHERE key_num_code =200101;
select SUM(output_quant) into output_q FROM traffic WHERE key_num_code =200101;
diff := input_q - output_q;
Final insert will be like:
INSERT INTO status (Status_comunal_waste) VALUES (diff);