how to count max, min, average, and median of quantity order - mysql

Question

I have a table called order_match which contain order_buyer_Id as the id of the transaction, createdby as the id of the buyer, and createdAt as the date when the transaction happened and quantity as the quantity of each order.
In this case, I want to count of the order (order_buyer_Id) for each buyer (createdby) and find out the maximum and the minumum count after that.
this is the example data:
+----------------+-----------+------------+--------+
| order_buyer_id | createdby | createdAt |quantity|
+----------------+-----------+------------+--------+
| 19123 | 19 | 2017-02-02 |0.4 |
| 193241 | 19 | 2017-02-02 |0.5
| 123123 | 20 | 2017-02-02 |1 |
| 32242 | 20 | 2017-02-02 |4
| 32434 | 20 | 2017-02-02 |3 |
+----------------+-----------+------------+---------
and if I run the query, the expected result is:
+-----+-----+---------+--------+
| max | min | average | median |
+-----+-----+---------+--------+
| 4 | 0.4 | 1,78 | 1 |
+-----+-----+---------+---------
This is the fiddle
http://www.sqlfiddle.com/#!9/d89772/15
and this is my query
SELECT MAX(quantity) AS max,
MIN(quantity) AS min,
AVG(quantity) AS average,
AVG(CASE WHEN rn IN (FLOOR((#tr+1)/2), FLOOR((#tr+2)/2)) THEN quantity END) AS median
FROM (
SELECT count,
#rn := #rn + 1 AS rn,
#tr := #rn AS tr
FROM (
SELECT COUNT(*) AS count
FROM order_match
GROUP BY order_buyer_Id
order by quantity
) o
CROSS JOIN (SELECT #rn := 0) init
) c

Answer 1

You are getting the error because quantity is not in your subquery.
Either you have join with your table again to get the quantity or you can include the quantity in you select (based on your sample data even group by with quantity gives the same result)
SELECT MAX(quantity) AS max,
MIN(quantity) AS min,
AVG(quantity) AS average,
AVG(CASE WHEN rn IN (FLOOR((#tr+1)/2), FLOOR((#tr+2)/2)) THEN quantity END) AS median
FROM (
SELECT count, quantity,
#rn := #rn + 1 AS rn,
#tr := #rn AS tr
FROM (
SELECT COUNT(*) AS count,Quantity
FROM order_match
GROUP BY order_buyer_Id,Quantity
order by quantity
) o
CROSS JOIN (SELECT #rn := 0) init
) c
SQL FIDDLE

Answer 2

SELECT t.max,t.min,t.average,0.00 AS 'Median'
FROM
(SELECT MAX(quantity) AS max,
MIN(quantity) AS min,
SUM(quantity)/COUNT(distinct created_by) AS average
FROM order_match)t
union
SELECT 0.00 AS 'max',0.00 AS 'min',0.00 AS 'Average',
((2*t1.average/3)+t1.mode) AS 'Median'
FROM (SELECT count(FLOOR(quantity)),IFNULL(FLOOR(quantity),min(quantity)) AS 'mode'
FROM order_match GROUP BY quantity HAVING
count(FLOOR(quantity))>1)t1