MYSQL trigger error (lots of function) - mysql

Question

i can't seem to found any fault on my code to make a trigger. ( i usually code using oracle, but i convert to my sql in this project, checked all the function and convert those that aren't available in mysql already)
here's the code :
CREATE TRIGGER `transaction_before_insert` BEFORE INSERT ON `transaction` FOR EACH ROW BEGIN
DECLARE TEMPKODE VARCHAR(12);
DECLARE TEMP VARCHAR(5);
TEMP:= CONCAT('T',DATE_FORMAT(NOW(),'%Y'));
SELECT CONCAT(TEMP, LPAD(NVL(MAX(CAST(SUBSTR(TRANSACTION_ID,5,5) AS UNSIGNED))+1,1),5,0))
FROM TRANSACTION INTO TEMPKODE
WHERE SUBSTR(TRANSACTION_ID,1,4) = TEMP;
NEW.TRANSACTION_ID := TEMPKODE;
END
EDIT 1:
i'm coding it from heidisql if there's any code difference, since i heard if i do it on mysql work bench i should use
SET variables
instead of directly
variables :=
the desired result is forex: T201600001
//T for transaction, 2016 i got it from dateformat, and the rest is choosing the biggest data from the database
it's a software for production planning so i'm making the transaction code

Answer 1

NVL, is a function built for you?, Oracle NVL function does not exist in MySQL (find its equivalent in MySQL), see IFNULL.
DELIMITER $$
BEGIN
DECLARE TEMPKODE VARCHAR(12);
DECLARE TEMP VARCHAR(5) DEFAULT CONCAT('T',DATE_FORMAT(NOW(),'%Y'));
-- OR: SET TEMP := CONCAT('T',DATE_FORMAT(NOW(),'%Y'));
-- TEMP := CONCAT('T',DATE_FORMAT(NOW(),'%Y'));
/*
SELECT CONCAT(TEMP,LPAD(NVL(MAX(CAST(SUBSTR(TRANSACTION_ID,5,5) AS UNSIGNED))+1,1),5,0))
FROM TRANSACTION INTO TEMPKODE
WHERE SUBSTR(TRANSACTION_ID,1,4) = TEMP;
*/
SELECT CONCAT(TEMP,LPAD(COALESCE(MAX(CAST(SUBSTR(TRANSACTION_ID,5,5) AS UNSIGNED))+1,1),5,0))
FROM TRANSACTION
WHERE SUBSTR(TRANSACTION_ID,1,4) = TEMP INTO TEMPKODE;
-- NEW.TRANSACTION_ID := TEMPKODE;
SET NEW.TRANSACTION_ID := TEMPKODE;
END$$
DELIMITER ;
UPDATE
You can simplify with the answer of #GordonLinoff:
SET NEW.TRANSACTION_ID := CONCAT(...);

Answer 2

You really do not need temporary variables for this operation (in either Oracle or MySQL). I think the following is the same logic:
BEGIN
select new.transactionid := CONCAT('T', YEAR(now()),
LPAD(COALESCE(MAX(SUBSTR(TRANSACTION_ID, 5, 5) + 1
), 1
), 5, 0)
from transaction t
where TRANSACTION_ID LIKE CONCAT(YEAR(now()), '%')
END;