Getting total average between dates - mysql

Question

I have a table named sales with the following format.
sale_id user_id sale_date sale_cost
j847bv-6ggd bd48ta36-cn5x 2017-01-10 15:43:12 30
vf87x2-15gr bd48ta36-cn5x 2017-01-05 13:41:16 60
3gfd7f-2cdd 8g4f5ccf-1fet 2017-01-15 14:10:12 100
4bgfd5-12vn 8g4f5ccf-1fet 2017-01-20 19:47:14 20
b58e32-bf87 8g4f5ccf-1fet 2017-01-20 17:35:13 15
bg87db-127g gr4gg1f4-3gbb 2017-01-20 12:26:15 80
How could I get the average amount that a user (user_d) spends within the first X amount of days since their first purchase? I don't want an average for every user, but a total average for all.
I know that I can get the average using select avg(sale_cost) but I'm unsure how to find out the average for a date period.

Answer 1

You can find average of total for each user within 10 days date range from intial sales date like this:
select avg(sale_cost)
from (
select sum(t.sale_cost) sale_cost
from your_table t
join (
select user_id, min(sale_date) start_date, date_add(min(sale_date), interval 10 day) end_date
from your_table
group by user_id
) t2 on t.user_id = t2.user_id
and t.sale_date between t2.start_date and t2.end_date
group by t.user_id
) t;
It finds the first sale_date and date 10 days after this for each user. Then joins it with the table to get total for each user within that range and then finally average of the above calculated totals.
Demo
If you want to find the average between overall first sale_date (not individual) and 10 days from it, use:
select avg(sale_cost)
from (
select sum(t.sale_cost) sale_cost
from your_table t
join (
select min(sale_date) start_date, date_add(min(sale_date), interval 10 day) end_date
from your_table
) t2 on t.sale_date between t2.start_date and t2.end_date
group by t.user_id
) t;
Demo

Answer 2

The between operator comes in handy whenever it comes to checking ranges
SELECT column_name(s)
FROM table_name
WHERE column_name BETWEEN value1 AND value2;
In this case value1 and value2 will be replaced by your dates using:
'2011-01-01 00:00:00' AND '2011-01-31 23:59:59'
or
sale_date AND DATE_ADD(OrderDate,INTERVAL 10 DAY)
The first way is faster and also the between values are inclusive.