get top and bottom 25th percentile average - mysql

Question

I have a table with list of employees and the number of units that they have sold.
I want to get the top 25 percentile Avg units sold and Bottom 25 percentile Avg units sold.
I have created a representation of my data SLQ Fiddle
I really have no idea how to start on this? All the examples i see are for SQL Server and not MySQL. Here is what i am thinking.
I want 25 percentile and cant limit to 25 items. Basically it would involve:
1) #_of_employees = The number of total employees.
2) #_of_employees_in_25_percentile = #_of_employees*0.25
3) Calculate the sum of the units sold by the top/bottom 25 percentile (limit #_of_employees_in_25_percentile)
4) Divide the sum by #_of_employees_in_25_percentile to get the average.
How can all this be done efficiently in MySQL?

Answer 1

This is a solution that uses a devious trick I learned from this question.
SELECT id, unit_sold, n * 100 / #total AS percentile
FROM (
SELECT id, unit_sold, #total := #total + unit_sold AS n
FROM mydata, (SELECT #total := 0) AS total
ORDER BY unit_sold ASC
) AS t
SQL Fiddle.

Answer 2

What about this?
SELECT
SUM(unit_sold) AS sum_tot, SUM(unit_sold)/count(id) AS average,
SUM(CASE WHEN percentile<25 THEN unit_sold ELSE 0 END) AS sum_top25,
SUM(CASE WHEN percentile<25 THEN 1 ELSE 0 END) AS count_top25,
SUM(CASE WHEN percentile<25 THEN unit_sold ELSE 0 END)/SUM(CASE WHEN percentile<25 THEN 1 ELSE 0 END) AS average_top25,
SUM(CASE WHEN percentile>75 THEN unit_sold ELSE 0 END) AS sum_bottom25,
SUM(CASE WHEN percentile>75 THEN 1 ELSE 0 END) AS count_bottom25,
SUM(CASE WHEN percentile>75 THEN unit_sold ELSE 0 END)/SUM(CASE WHEN percentile>75 THEN 1 ELSE 0 END) AS average_bottom25
FROM
(SELECT
id, unit_sold, c * 100 / #counter AS percentile
FROM
(SELECT
m.*, #counter:=#counter+1 AS c
FROM
(SELECT #counter:=0) AS initvar, mydata AS m
ORDER BY unit_sold desc
) AS t
WHERE
c <= (25/100 * #counter)
OR c >= (75/100 * #counter)
) AS t2
Output:
SUM_TOT AVERAGE SUM_TOP25 COUNT_TOP25 AVERAGE_TOP25 SUM_BOTTOM25 COUNT_BOTTOM25 AVERAGE_BOTTOM25
850 283.3333 500 1 500 350 2 175
See SQL Fiddle.
The idea is to use the MySQL: LIMIT by a percentage of the amount of records? solution to get the percentiles. Based on that (and on pdw answer) we create an output in which we just show the top 25% and bottom 75%.
Finally, we count and sum to get the values you requested.
Note this runs on top of the command:
SELECT
id, unit_sold, c * 100 / #counter AS percentile
FROM
(SELECT
m.*, #counter:=#counter+1 AS c
FROM
(SELECT #counter:=0) AS initvar, mydata AS m
ORDER BY unit_sold desc
) AS t
WHERE
c <= (25/100 * #counter)
OR c >= (75/100 * #counter)
Whose output is:
ID UNIT_SOLD PERCENTILE
d 500 20
a 250 80
e 100 100

Answer 3

How about going with this logic:
Select all, order by percentile (DESC), limit to 25
Select all, order by percentile (ASC), limit to 25
Is this the type of logic you're looking for ?
Sample queries:
$q1 = mysql_query(SELECT * FROM table_name ORDER BY percentile DESC LIMIT 25)
$q2 = mysql_query(SELECT * FROM table_name ORDER BY percentile ASC LIMIT 25)