How to create trigger that calculate delta between two tables? - postgresql

Question

I have three tables. I need create trigger that calculate price difference between two tables and insert it to result table.
Algorithm is next:
If we inserting value in table2 we should check if table1 have same id, get start_price from it, calculate difference, and insert result in result table.
For example if we are insering (assume table1 is already filled) :
INSERT INTO table2 VALUES (1, 135);
we should get in result table:
1 3.58%
caculation logic: 100-((135/140) * 100) = 3.58%
I can't figure out how to do this math operation inside trigger:
CREATE TABLE table1
(
id integer NOT NULL,
start_price integer NOT NULL,
CONSTRAINT table1_pkey PRIMARY KEY (id)
);
CREATE TABLE table2
(
id integer NOT NULL,
end_price integer NOT NULL
);
CREATE TABLE result
(
id integer NOT NULL,
result_price decimal NOT NULL
);
INSERT INTO table1 VALUES (1, 140);
INSERT INTO table1 VALUES (2, 230);
INSERT INTO table1 VALUES (3, 70);
trigger:
CREATE OR REPLACE FUNCTION delta_percent_calc() RETURNS TRIGGER AS
$BODY$
BEGIN
INSERT INTO
result(id,name)
-- some logic here. seems that INSERT or UPDATE should be here because table1 and table2 price can be changed with update
RETURN new;
END;
$BODY$
language plpgsql;
UPD:. it's seems that I get it work with follow solution:
CREATE OR REPLACE FUNCTION delta_percent_calc() RETURNS TRIGGER AS
$BODY$
DECLARE
s_price integer;
BEGIN
select start_price into s_price from table1 where id = new.id;
INSERT INTO
result (id,result_price)
VALUES(new.id, round(100 - ( (new.end_price::numeric/s_price::numeric) * 100), 4 ) );
ON CONFLICT (id) DO UPDATE
SET result_price = round(100 - ( (new.end_price::numeric/s_price::numeric) * 100), 4 );
RETURN new;
END;
$BODY$
language plpgsql;
CREATE OR REPLACE TRIGGER trig_percent_calc
AFTER INSERT ON table2
FOR EACH ROW
EXECUTE PROCEDURE delta_percent_calc();