MySql查找由另一个分组的列的中位数，而不是整列的中位数

Question

上下文：

我正在尝试进行一系列市场交易，并确定每种物品类型实际移动的金额。这几乎是我第一次尝试 MySql，所以查询很丑，但以下几乎可以工作：

SELECT types.typename,
       averages.type,
       averages.price,
       movement.sold,
       ( averages.price * movement.sold ) AS value
FROM   (SELECT type,
               Round(Avg(price)) AS price
        FROM   orders
        GROUP  BY type) AS averages
       INNER JOIN (SELECT type,
                          ( startingvolume - currentvolume ) AS sold
                   FROM   (SELECT type,
                                  Sum(volume)        AS currentVolume,
                                  Sum(volumeentered) startingVolume
                           FROM   orders
                           GROUP  BY type) AS movement
                   WHERE  ( startingvolume - currentvolume ) > 10000
                   ORDER  BY sold) AS movement
               ON averages.type = movement.type
       INNER JOIN invtypes AS types
               ON types.typeid = averages.type
ORDER  BY value DESC
LIMIT  10 ;

-

+------------------------------------+-------+---------+------------+------------------+
| typeName                           | type  | price   | sold       | value            |
+------------------------------------+-------+---------+------------+------------------+
| Dirt                               |    34 | 1904767 | 2670581874 | 5086836224393358 |
| Light Wood                         |  2629 |   42999 |    2756595 |     118530828405 |
| Dark Wood                          | 24509 |   47344 |    1107771 |      52446310224 |
| Stone                              | 21922 |   18386 |    1505884 |      27687183224 |
| Grass                              |   238 |    5643 |    4554470 |      25700874210 |
| Paper                              |  3814 |   25635 |     861006 |      22071888810 |
| Iron                               |  3699 |  320270 |      58833 |      18842444910 |
| Ink                                | 16275 |    8552 |    2200545 |      18819060840 |
| Loam                               |  2679 |    5759 |    2608771 |      15023912189 |
| Copper                             |   672 |  904612 |      14989 |      13559229268 |
+------------------------------------+-------+---------+------------+------------------+

上述数据的问题在于原始市场数据不可避免地被异常值破坏，如下所示：

select type, price from orders where type = 34 order by price desc limit 10;

-

+------+-----------+
| type | price     |
+------+-----------+
|   34 | 200000000 |
|   34 |     15.99 |
|   34 |     12.06 |
|   34 |        10 |
|   34 |      7.67 |
|   34 |       7.5 |
|   34 |       7.3 |
|   34 |      7.17 |
|   34 |       7.1 |
|   34 |      7.06 |
+------+-----------+

核心问题：

99%的市场数据是干净的，但异常值破坏了平均值，MySql似乎没有中值功能。我找到了几个如何找到整个列的中位数的示例，但我需要每个项目的中位数。

如何确定每个项目的中位数而不是每个项目的平均值，或者在运行主查询之前有效地清理这些异常值的数据？

注意： 我尝试通过 std 省略结果，但商品的价格从 $17 到 $10B 不等，而无论价格范围如何，偏差仍然相对较低。

Answer 1

我不会触及您的原始查询，因为它非常复杂，但您可以做的一个选择是使用子查询来删除任何统计异常值。例如，如果您想从orders 表中删除任何离您可以使用的平均值相差两个标准差以上的异常值：

SELECT t1.type,
       t1.price
FROM orders t1
INNER JOIN
(
    SELECT type,
           AVG(price) AS AVG,
           STD(price) AS STD
    FROM orders
    GROUP BY type
) t2
    ON t1.type = t2.type
WHERE t1.price < ABS(2*t2.STD - t2.AVG)  -- any value more than 2 standard devations
                                         -- away from the mean is discarded

演示在这里：

SQLFiddle