Oracle trigger: SELECT INTO, no data found - sql

Question

I have the following table that describes which chemical elements each planet is composed of using percentage.
CREATE TABLE elem_in_planet
(
id_planet INTEGER,
element_symbol CHAR(2),
percent_representation NUMBER CONSTRAINT NN_elem_in_planet NOT NULL,
CONSTRAINT PK_elem_in_planet PRIMARY KEY (id_planet, element_symbol),
CONSTRAINT FK_planet_has_elem FOREIGN KEY (id_planet) REFERENCES planet (id_planet),
CONSTRAINT FK_elem_in_planet FOREIGN KEY (element_symbol) REFERENCES chemical_element (element_symbol)
);
I'm trying to make a trigger that warns users when they add a new element to a planet and the sum of elements in that planet exceeds 100%. I came up with this.
CREATE OR REPLACE TRIGGER elem_in_planet_check
AFTER INSERT OR UPDATE ON elem_in_planet
FOR EACH ROW
DECLARE
sum_var NUMBER;
PRAGMA autonomous_transaction;
BEGIN
SELECT SUM(percent_representation)
INTO sum_var
FROM elem_in_planet
WHERE id_planet = :NEW.id_planet
GROUP BY id_planet;
EXCEPTION
WHEN NO_DATA_FOUND THEN
sum_var := 0;
IF sum_var > 100 THEN
DBMS_OUTPUT.put_line('WARNING: Blah blah.');
END IF;
END;
/
This code seems to throw the NO_DATA_FOUND exception every single time, even though I have inserted test data and when I run the SQL query alone, it works as expected.
I'm new to this and don't understand what I'm doing wrong.
Thank you for any advice.

Answer 1

You have NOT inserted the row into the table, 2 reasons.
The trigger runs as part of the insert statement which has not
completed. So the row does not exist.
You specified "PRAGMA autonomous_transaction" (AKA the create
untraceable bug here statement), did you previously get a mutating
table exception. So you cannot see any data inserted/updated/deleted
by the current transaction.Further if an error did occur the row would still be inserted as you did not raise an error or re-raise the existing error. I suggest you familiarize yourself with the PLSQL block structure. For now you may want to try:
You could use an after statement trigger or after statement section of compound trigger to make this test, do raise_application_error if sum > 100;
BTW as it stands your "if on sum_var > 100" runs only when an error occurs. Anything after the "EXCEPTION" and before END for that block runs only when a error occurs.
create or replace trigger elem_in_planet_check
after insert or update on elem_in_planet
declare
error_detected boolean := False;
begin
for planet in
(
select id_planet, sum_var
from (select id_planet, sum(percent_representation) sum_var
from elem_in_planet
group by id_planet
)
where sum_var > 100
)
loop
dbms_output.put_line('Planet ' || planet.id_planet || ' at '|| planet.sum_var || '% resources exceed 100%');
error_detected:= True;
end loop;
if error_detected then
Raise_application_error('-20001', 'Planet resources cannot exceed 100%');
end if;
end elem_in_planet_check;