如何在 mysql 5.7 中正确计算中位数

Question

这是我的小提琴https://dbfiddle.uk/?rdbms=mysql_5.7&fiddle=7946871d9c25cd8914353c70fde1fe8d

所以这是我的查询 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 |
    +--------+---------+

基于该表，我想通过此查询找出最大值（itung）、最小值（itung）和中值（itung）

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 |
+------------+------------+-----------------------+--------+

您知道对中位数的查询是错误的，因为中位数应该是 1,5 而不是 2。我在中位数查询中的错误在哪里？

Answer 1

您有 ROUND() 将报告的中位数四舍五入为整数。如果您不想要，请将其删除：

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

或保留四舍五入并添加小数位数以四舍五入，此处为 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

请注意，只有在行数不多的情况下，才能从 GROUP_CONCAT 逗号分隔的所有值列表中查找中值，因为 GROUP_CONCAT 将在 @@group_concat_max_len 处截断，在 MySQL 上默认为 1024 个字符或在 10.2 之前的 MariaDB 上。