Query average with nested subquery - mysql

Question

I cannot figure out how to calculate the running average per customer up until each month.
I tried to write it in one big query using subqueries, and also joins with no luck
Here is the query I tried with a subquery:
SELECT
date_format(z1.ServiceDate, '%y-%b') as months,
(
SELECT
AVG(cc.total) + 1 AS 'avg'
FROM
(
SELECT
z.Customer_ID,
COUNT(z.BookingId) 'total'
from
Orders z
where
YEAR(z.ServiceDate) <= YEAR(z1.months) AND
MONTH(z.ServiceDate) <= MONTH(z1.months)
GROUP BY
z.Customer_ID
) cc
)
from
Orders z1
GROUP BY
YEAR(z1.ServiceDate),
MONTH(z1.ServiceDate)
I also tried to join these two queries with no luck:
SELECT date_format(Orders.ServiceDate, '%y-%b') from Orders
GROUP BY YEAR(Orders.ServiceDate), month(Orders.ServiceDate)
Could not join it with this one:
(
SELECT AVG(cc.total) + 1 AS 'avg' FROM (
SELECT Orders.Customer_ID as 'c',
COUNT(BookingId) 'total' from Orders
where year(Orders.ServiceDate) <= '2019' and month(Orders.ServiceDate)
<= '01'
GROUP BY Orders.Customer_ID
) cc
)
where '2019' and '01' would be taken from the first query.
Here is my test schema:
CREATE TABLE IF NOT EXISTS `orders` (
`BookingId` INT(6) NOT NULL,
`ServiceDate` DATETIME NOT NULL,
`Customer_ID` varchar(1) NOT NULL,
PRIMARY KEY (`BookingId`)
) DEFAULT CHARSET=utf8;
INSERT INTO `orders` (`BookingId`, `ServiceDate`, `Customer_ID`) VALUES
('1', '2019-01-03T12:00:00', '1'),
('2', '2019-01-04T12:00:00', '2'),
('3', '2019-01-12T12:00:00', '2'),
('4', '2019-02-03T12:00:00', '1'),
('5', '2019-02-04T12:00:00', '2'),
('6', '2019-02-012T12:00:00', '3');
I was expecting something like this for all months
month AVG
19-Jan 1.5
19-Feb 2
...
...
The dots is there only to show that there is much many more months in my original dataset.
For January, there was 3 bookings and two Customer_ID's. Therefore the average for bookings up until that month was 1.5. Up until February, There has been 6 bookings, and 3 Customer_IDs. Therefore the new average is 2

Answer 1

Join a subquery that returns the distinct months to the table and aggregate:
SELECT d.month,
COUNT(o.bookingid) / COUNT(DISTINCT o.customer_id) avg
FROM (
SELECT DISTINCT
EXTRACT(YEAR_MONTH FROM servicedate) yearmonth,
DATE_FORMAT(servicedate, '%y-%b') month
FROM orders
) d INNER JOIN orders o
ON EXTRACT(YEAR_MONTH FROM o.servicedate) <= d.yearmonth
GROUP BY d.yearmonth, d.month
See the demo.
Results:
| month | avg |
| ------ | --- |
| 19-Jan | 1.5 |
| 19-Feb | 2 |