MySQL - 行的底部（X）百分比限制

Question

这是我的表结构：

+-------+--------+----------+
| item  | price  | quantity |
+-------+--------+----------+
| 22452 | 579150 |        4 |
| 34664 | 334425 |        7 |
| 32249 | 204750 |        3 |
| 39970 |  97500 |        5 |
| 36907 | 116415 |        6 |
|  4338 | 207451 |       17 |
| 23425 | 388050 |        4 |
| 23427 | 532350 |       14 |
| 76080 | 180000 |        6 |
| 76076 | 400000 |        4 |
+-------+--------+----------+

项目不是唯一的，每个项目可能有 1 到几千行不等，因此我按项目对结果进行分组。我当前的查询如下：

SELECT  item AS id,
        COUNT(item) as total,
        ROUND(AVG(price/quantity)) AS mean,
        ROUND(MIN(price/quantity)) AS cheapest
FROM `data`
GROUP BY item;

除了这 4 个结果之外，我还想计算 (price/quantity) 值的底部 15% 行的平均价格（不是

下面的答案更新

使用下面的@GordonLinoff 答案让我基本上一路走来。但是，我确实遇到了两个问题。最大的一个是 @rn 变量没有重置，这导致它保持递增，随后只有第一行项目被包含在内。第二个是在表中出现次数的 15%

SELECT item AS id,
       COUNT(item) as total,
       ROUND(AVG(price/quantity)) AS mean,
       ROUND(MIN(price/quantity)) AS cheapest,
       ROUND(avg(case when rn <= IF(cnt * 0.15 < 1, cnt, cnt * 0.15) then price/quantity end)) as Cheapest15Percent 
FROM
    (SELECT d.*, cnt, IF(@item = d.item, @rn := @rn + 1, if(@item := d.item, @rn := 1, 1)) as rn
    FROM `data` d LEFT JOIN
        (SELECT item, COUNT(*) cnt FROM `udata` GROUP BY item) di
    ON d.item = di.item CROSS JOIN
       (SELECT @rn := 0, @item := -1) vars
    ORDER BY d.item, d.price/d.quantity) d
GROUP BY d.item;

Answer 1

这假设您想要每件商品的平均 15% 最便宜。

以下查询枚举每个项目的行并获取总行数：

  select d.*, cnt,
         if(@item = item, @rn := @rn + 1, if(@item := item, 1, 1)) as rn
  from `data` d left join
       (select item, count(*) cnt
        from data
        group by item
       ) di
       on d.item = di.item cross join
       (select @rn := 0, @item := -1) vars
  order by item, price/quantity;

然后您基本上可以将其插入到您的查询中并进行条件聚合：

SELECT item AS id,
       COUNT(item) as total,
       ROUND(AVG(price/quantity)) AS mean,
       ROUND(MIN(price/quantity)) AS cheapest,
       avg(case when rn <= cnt * 0.15 then price/quantity end) as Cheapest15Percent
FROM (select d.*, cnt,
             if(@item = item, @rn := @rn + 1, if(@item := item, 1, 1)) as rn
      from `data` d left join
           (select item, count(*) cnt
            from data
            group by item
           ) di
           on d.item = di.item cross join
           (select @rn := 0, @item := -1) vars
      order by item, price/quantity
     ) d
GROUP BY item;