Set Insert trigger to store max value in another column - sql

Question

This is what my table looks like:
student_id
subject_id
total
max
101
1
6
102
2
5
103
1
9
101
1
10
103
2
2
104
1
7
I want the "max" column to be automatically populated when the total is inserted.
Expected Output:
student_id
subject_id
total
max
101
1
10
10
102
2
5
7
103
1
9
10
101
1
8
10
103
2
2
7
104
1
7
10
I will like to create a trigger for this.
This is my SELECT statement which works fine but how do I put it in a trigger?
WITH CTE AS (SELECT `subject_id`,MAX(`total`) AS MaxTotal
FROM results
GROUP BY `subject_id`
)
SELECT results.*,CTE.MaxTotal
FROM results
JOIN CTE ON results.`subject_id` = CTE.`subject_id`;
I did this but I got a plethora of errors
CREATE TRIGGER `max_score_before_INSERT` BEFORE INSERT ON `results`
FOR EACH ROW
SET NEW.max = (WITH CTE AS (SELECT `subject_id`,MAX(`NEW.total`) AS MaxTotal
FROM results
GROUP BY `subject_id`
)
SELECT results.*,CTE.MaxTotal
FROM results
JOIN CTE ON results.`subject_id` = CTE.`subject_id`
);

Answer 1

You can't reference "new" inside a BEFORE trigger type. You may want to first
insert the row, then update the column with an AFTER trigger type:
CREATE TRIGGER `max_score_before_INSERT` AFTER INSERT ON `results`
FOR EACH ROW
UPDATE <your_table_name>
INNER JOIN (SELECT subject_id,
MAX(total) AS total_max
FROM <your_table_name>
GROUP BY subject_id) cte
ON <your_table_name>.subject_id = cte.subject_id
SET <your_table_name>.max = cte.total_max;
Note that this approach will update the previous columns too, as if you insert a new field that becomes the max, you may want to update the already existing rows too. If not, then you can use a condition inside the UPDATE statement:
CREATE TRIGGER `max_score_before_INSERT` AFTER INSERT ON `results`
FOR EACH ROW
UPDATE <your_table_name>
INNER JOIN (SELECT subject_id,
MAX(total) AS total_max
FROM <your_table_name>
GROUP BY subject_id) cte
ON <your_table_name>.subject_id = cte.subject_id
SET <your_table_name>.max = cte.total_max;
WHERE <your_table_name>.student_id = NEW.student_id
AND <your_table_name>.subject_id = NEW.subject_id;