How to calculate the median category wise in mysql - mysql

Question

I need to find median of total scores region wise. I have got the solution after trial and error on data but the query is not in a optimized way. i need a efficient mysql query for this problem.
#Thanks for the solutions#
Edit: first exam has to be filter from assessment table and second total_score needs to be summed for all subject of each student using studentassessment table. Then finally region wise median needs to be calculated.
SELECT region,
Avg(total_score) AS median
FROM (SELECT row_num,
region,
total_score,
region_cnt
FROM (SELECT Row_number()
OVER (
partition BY region
ORDER BY total_score) AS row_num,
region,
total_score,
Count(region)
OVER (
partition BY region) AS region_cnt
FROM (SELECT i.region AS region,
Sum(S.score) AS total_score
FROM tredence.assesment A
INNER JOIN tredence.studentassessment S
ON A.id_assessment = S.id_assessment
INNER JOIN tredence.studentinfo i
ON i.id_student = S.id_student
WHERE A.assessment = 'Exam'
GROUP BY S.id_student,
i.region
ORDER BY region,
total_score) t) r
GROUP BY 1,
2,
3,
4
HAVING row_num IN ( Floor(region_cnt / 2), Ceil(region_cnt / 2) )) z
GROUP BY region
ORDER BY median DESC ```
tables and columns:
|Assessments |student_info|student_assessment|
|---------------|------------|------------------|
|course_code |course_code |id_assessment |
|batch_code |batch_code |id_student |
|id_assessments |id_student |date_submitted |
|assessment_type|gender |is_banked |
|date |region |score |
Output:
|Region |Median|
|-------------|------|
|North Region | 82 |
|London Region| 80 |
|Scotland | 80 |
|Ireland | 76 |

Answer 1

Assuming you reduce the set to the following. Note: id_student isn't required at this point in the calculation.
CREATE TABLE tscores (
id int primary key auto_increment
, region int
, id_student int
, total_score int
, index (region, total_score)
);
INSERT INTO tscores (region, id_student, total_score) VALUES
(1, 1000, 40)
, (1, 1001, 50)
, (1, 1002, 30)
, (1, 1003, 90)
, (2, 1101, 50)
, (2, 1102, 51)
, (2, 1103, 55)
;
SQL and Result:
WITH cte1 AS (
SELECT region, total_score
, ((COUNT(*) OVER (PARTITION BY region) + 1) / 2) AS n
, ROW_NUMBER() OVER (PARTITION BY region ORDER BY total_score) AS rn
FROM tscores AS t
)
SELECT region
, truncate(AVG(total_score), 2) AS med_score
FROM cte1 AS t
WHERE rn IN (ceil(n), floor(n))
GROUP BY region
;
+--------+-----------+
| region | med_score |
+--------+-----------+
| 1 | 45.00 |
| 2 | 51.00 |
+--------+-----------+
2 rows in set (0.004 sec)
Still not quite enough detail. But here's SQL that runs against your schema, minus the typos I think you had in your SQL:
WITH tscores AS (
SELECT i.region AS region
, Sum(S.score) AS total_score
FROM tredence.assessments A
JOIN tredence.studentassessment S
ON A.id_assessment = S.id_assessment
JOIN tredence.studentinfo i
ON i.id_student = S.id_student
WHERE A.assessment = 'Exam'
GROUP BY S.id_student
, i.region
)
, cte1 AS (
SELECT region, total_score
, ((COUNT(*) OVER (PARTITION BY region) + 1) / 2) AS n
, ROW_NUMBER() OVER (PARTITION BY region ORDER BY total_score) AS rn
FROM tscores AS t
)
SELECT region
, truncate(AVG(total_score), 2) AS med_score
FROM cte1 AS t
WHERE rn IN (ceil(n), floor(n))
GROUP BY region
;