MySQL - Limit by bottom (X) percent of rows - mysql

Question

Here's my table structure:
+-------+--------+----------+
| 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 |
+-------+--------+----------+
Item is not unique and there could be anywhere from 1 to a few thousand rows for each item, so I'm grouping by item for the results. My current query is the following:
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;
In addition to these 4 results, I would like to calculate the average price of the bottom 15% of rows of the (price/quantity) value (not < 0.15*MAX(price/quantity) but 0.15*total, ordered by (price/quantity) ASC). The solution I thought of involved temporary tables using the count of that item as the limiter, but I'd highly prefer it to be a single query if possible. I'm sure I'll need a subquery in here, but I'm unsure of how to go about getting the count for that particular item and then limiting by 15% of that result.
UPDATE WITH ANSWER FROM BELOW
Using #GordonLinoff answer below got me basically all the way there. I did run into two issues, however. The biggest one was the #rn variable wasn't resetting, which was causing it to keep increment and subsequently only the first row of the items was getting included. The second was any item where 15% of the number of times it appears in the table is < 1, NULL was being returned. The corrections were minor and I've included the final query I used below:
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

This assumes that you want the average of the cheapest 15% for each item.
The following query enumerates the rows for each item and gets the total rows:
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;
You can then basically plug this into your query and do conditional aggregation:
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;