Changing an update trigger to insert trigger in MySQL

Question

I am a beginner and studying about trigger to calculate grade of students from this link. Student_Marks Trigger The query for before update trigger works fine. I was trying to change the trigger a little bit. I want to calculate the Grade after inserting marks of a student. Hence, after trigger instead of update. I got stuck.

My query is as follows:

DELIMITER 
$$
CREATE TRIGGER `student_marks` 
AFTER INSERT 
ON student FOR EACH ROW

BEGIN 
SET NEW.Total =NEW.SUB1+NEW.SUB2+NEW.SUB3+NEW.SUB4+NEW.SUB5; 
SET NEW.Percentage = NEW.Total/5;
IF NEW.Percentage >=90 THEN
SET NEW.Grade = 'EXCELLENT';
ELSEIF NEW.Percentage>=75 AND NEW.Percentage<90 THEN
SET NEW.Grade = 'VERY GOOD';
ELSEIF NEW.Percentage>=60 AND NEW.Percentage<75 THEN
SET NEW.Grade = 'GOOD';
ELSEIF NEW.Percentage>=40 AND NEW.Percentage<60 THEN
SET NEW.Grade = 'AVERAGE';
ELSE SET NEW.Grade = 'NOT PROMOTED';
END IF;
END;
$$  

Answer 1

You have to use BEFORE INSERT instead of AFTER INSERT to save the calculated value of the new row. You can't affect the row AFTER INSERT because the row is already saved.

DELIMITER $$

CREATE TRIGGER `student_marks` 
BEFORE INSERT ON student_marks FOR EACH ROW
BEGIN 
    SET NEW.Total =NEW.SUB1+NEW.SUB2+NEW.SUB3+NEW.SUB4+NEW.SUB5; 
    SET NEW.Percentage = NEW.Total/5;

    IF NEW.Percentage >=90 THEN
        SET NEW.Grade = 'EXCELLENT';
    ELSEIF NEW.Percentage>=75 AND NEW.Percentage<90 THEN
        SET NEW.Grade = 'VERY GOOD';
    ELSEIF NEW.Percentage>=60 AND NEW.Percentage<75 THEN
        SET NEW.Grade = 'GOOD';
    ELSEIF NEW.Percentage>=40 AND NEW.Percentage<60 THEN
        SET NEW.Grade = 'AVERAGE';
    ELSE 
        SET NEW.Grade = 'NOT PROMOTED';
    END IF;
END;
$$  

You also need to change the table name for the TRIGGER (use INSERT ON student_marks instead of INSERT ON student).

demo on dbfiddle.uk