how to count median properly in mysql 5.7 - mysql

Question

this is my fiddle https://dbfiddle.uk/?rdbms=mysql_5.7&fiddle=7946871d9c25cd8914353c70fde1fe8d
so this is my queries
select count(user_id) as itung, user_Id from
(SELECT t1.user_id,
t1.createdAt cretecompare1,
t2.createdAt cretecompare2,
DATEDIFF(t2.createdAt, t1.createdAt) diff
-- table for a transaction
FROM test t1
-- table for prev. transaction
JOIN test t2 ON t1.user_id = t2.user_id
AND t1.createdAt < t2.createdAt
AND 7 NOT IN (t1.status_id, t2.status_id)
JOIN (SELECT t3.user_id
FROM test t3
WHERE t3.status_id != 7
GROUP BY t3.user_id
HAVING SUM(t3.createdAt < '2020-04-01') > 1
AND SUM(t3.createdAt BETWEEN '2020-02-01' AND '2020-04-01')) t4 ON t1.user_id = t4.user_id
WHERE NOT EXISTS (SELECT NULL
FROM test t5
WHERE t1.user_id = t5.user_id
AND t5.status_id != 7
AND t1.createdAt < t5.createdAt
AND t5.createdAt < t2.createdAt)
HAViNG cretecompare2 BETWEEN '2020-02-01' AND '2020-04-01') aa
group by user_Id
output table:
+--------+---------+
| itung | user_Id |
+--------+---------+
| 1 | 13 |
| 2 | 14 |
+--------+---------+
based on that table i want to find out the max(itung), min(itung), and the median(itung), with this query
select max(itung), min(itung), format(avg(itung), 2), IF(count(*)%2 = 1, CAST(SUBSTRING_INDEX(SUBSTRING_INDEX( GROUP_CONCAT(itung ORDER BY itung SEPARATOR ',')
, ',', 50/100 * COUNT(*)), ',', -1) AS DECIMAL), ROUND((CAST(SUBSTRING_INDEX(SUBSTRING_INDEX
( GROUP_CONCAT(itung ORDER BY itung SEPARATOR ','), ',', 50/100
* COUNT(*) + 1), ',', -1) AS DECIMAL) + CAST(SUBSTRING_INDEX(SUBSTRING_INDEX
( GROUP_CONCAT(itung ORDER BY itung SEPARATOR ','), ',', 50/100
* COUNT(*)), ',', -1) AS DECIMAL)) / 2)) as median from
(select count(user_id) as itung, user_Id from
(SELECT t1.user_id,
t1.createdAt cretecompare1,
t2.createdAt cretecompare2,
DATEDIFF(t2.createdAt, t1.createdAt) diff
-- table for a transaction
FROM test t1
-- table for prev. transaction
JOIN test t2 ON t1.user_id = t2.user_id
AND t1.createdAt < t2.createdAt
AND 7 NOT IN (t1.status_id, t2.status_id)
JOIN (SELECT t3.user_id
FROM test t3
WHERE t3.status_id != 7
GROUP BY t3.user_id
HAVING SUM(t3.createdAt < '2020-04-01') > 1
AND SUM(t3.createdAt BETWEEN '2020-02-01' AND '2020-04-01')) t4 ON t1.user_id = t4.user_id
WHERE NOT EXISTS (SELECT NULL
FROM test t5
WHERE t1.user_id = t5.user_id
AND t5.status_id != 7
AND t1.createdAt < t5.createdAt
AND t5.createdAt < t2.createdAt)
HAViNG cretecompare2 BETWEEN '2020-02-01' AND '2020-04-01') aa
group by user_Id) ab
output table:
+------------+------------+-----------------------+--------+
| max(itung) | min(itung) | format(avg(itung), 2) | median |
+------------+------------+-----------------------+--------+
| 2 | 1 | 1.50 | 2 |
+------------+------------+-----------------------+--------+
you know that's wrong query for the median because median should be 1,5 not 2. where my wrong at in my median query?

Answer 1

You have ROUND() there to round the reported median to an integer. If you don't want that, remove it:
select max(itung), min(itung), format(avg(itung), 2), IF(count(*)%2 = 1, CAST(SUBSTRING_INDEX(SUBSTRING_INDEX( GROUP_CONCAT(itung ORDER BY itung SEPARATOR ',') , ',', 50/100 * COUNT(*)), ',', -1) AS DECIMAL), (CAST(SUBSTRING_INDEX(SUBSTRING_INDEX ( GROUP_CONCAT(itung ORDER BY itung SEPARATOR ','), ',', 50/100 * COUNT(*) + 1), ',', -1) AS DECIMAL) + CAST(SUBSTRING_INDEX(SUBSTRING_INDEX ( GROUP_CONCAT(itung ORDER BY itung SEPARATOR ','), ',', 50/100 * COUNT(*)), ',', -1) AS DECIMAL)) / 2) as median
or leave in the round and add a number of decimal places to round to, here 3:
select max(itung), min(itung), format(avg(itung), 2), IF(count(*)%2 = 1, CAST(SUBSTRING_INDEX(SUBSTRING_INDEX( GROUP_CONCAT(itung ORDER BY itung SEPARATOR ',') , ',', 50/100 * COUNT(*)), ',', -1) AS DECIMAL), ROUND((CAST(SUBSTRING_INDEX(SUBSTRING_INDEX ( GROUP_CONCAT(itung ORDER BY itung SEPARATOR ','), ',', 50/100 * COUNT(*) + 1), ',', -1) AS DECIMAL) + CAST(SUBSTRING_INDEX(SUBSTRING_INDEX ( GROUP_CONCAT(itung ORDER BY itung SEPARATOR ','), ',', 50/100 * COUNT(*)), ',', -1) AS DECIMAL)) / 2, 3)) as median
Be aware that looking up the median values from a GROUP_CONCAT comma-separated list of all the values only works if there are not too many rows, since GROUP_CONCAT will be truncated at ##group_concat_max_len, which defaults to 1024 characters on MySQL or on MariaDB before 10.2.