Get max score and store in another column - mysql

Question

This is my table:
student_id
subject_id
total
max
101
1
90
102
2
80
103
1
95
101
1
82
103
2
76
104
1
95
101
2
71
I want to get the max total in a particular subject and store it in another column whenever the total column is inserted or updated.
This is what I want the table to look like:
student_id
subject_id
total
max
101
1
90
95
102
2
80
80
103
1
95
95
101
1
82
95
103
2
76
80
104
1
95
95
101
2
71
80
I have tried this query but it doesn't add the max scores in each subject against all the student_id.
SELECT MAX(`total`) AS highest
FROM results
GROUP BY student_id
I suppose I should use a trigger for this but the normal query is also okay by me.

Answer 1

If your column does not yet exist in your table, you need to add it with an ALTER statement.
ALTER TABLE <your_table_name> ADD max INT;
Then you can first select the "max" value for each "subject_id" using an aggregation, then use it inside the UPDATE statement exploiting a JOIN operation:
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;
Check the demo here.

Answer 2

Assuming you are using MySQL 8+.
Ideally, instead of storing MaxTotal data into each column, you should get such the data while selecting like this:
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;
However, if you still need to update it anyway, use UPDATE with JOIN
WITH CTE AS (SELECT subject_id,MAX(total) AS MaxTotal
FROM results
GROUP BY subject_id
)
UPDATE results
JOIN CTE ON results.subject_id = CTE.subject_id
SET results.MaxTotal = CTE.MaxTotal;
Output after the update:
student_id
subject_id
total
MaxTotal
101
1
90
95
102
2
80
80
103
1
95
95
101
1
82
95
103
2
76
80
104
1
95
95
101
2
71
80
See this db<>fiddle.