mysql function variable set not working - mysql

Question

I am writing a function that should return a floating value.
BEGIN
DECLARE due_amount DECIMAL(9,2);
SET due_amount = (SELECT due_amount FROM (
SELECT id, MAX(date), due_amount, user_id
FROM lunch_transaction
GROUP BY user_id
HAVING user_id = user) l);
IF due_amount IS NULL THEN
SET due_amount = 0.00;
END IF;
RETURN due_amount;
END
The function only returns value 0.00 even though the value should be something else.
Running only this query :
(SELECT due_amount FROM (
SELECT id, MAX(date), due_amount, user_id
FROM lunch_transaction
GROUP BY user_id
HAVING user_id = user) l);
is giving the correct output though.
How should I set the query's output to the variable?

Answer 1

It is really bad practice to use variable name that can conflict with column names. Also, the subquery seems very unnecessary. I would try something more like this:
BEGIN
DECLARE v_due_amount DECIMAL(9,2);
SELECT v_due_amount := l.due_amount
FROM lunch_transaction l
WHERE l.user_id = in_user; -- I'm guessing `user` is also a parameter
IF v_due_amount IS NULL THEN
SET v_due_amount = 0.00;
END IF;
RETURN v_due_amount;
END;
Your version has an aggregation function in the subquery. This makes no sense, because due_amount is not the argument of an aggregation function. This logic should perhaps be:
SELECT v_due_amount := SUM(l.due_amount)
FROM lunch_transaction l
WHERE l.user_id = in_user; -- I'm guessing `user` is also a variable