date_add(min(visited_on), interval 6 day)
means that add 6 days to min(visited_on)
The syntax:
DATE_ADD(date,INTERVAL expr type)
# Write your MySQL query statement below
select visited_on, (select sum(amount) from Customer where visited_on between date_sub(c.visited_on, interval 6 day) and c.visited_on) amount,
round(
(select sum(amount)/7 from Customer where visited_on between date_sub(c.visited_on, interval 6 day) and c.visited_on),2) average_amount from Customer c
where visited_on >= (
select date_add(min(visited_on), interval 6 day) from Customer
) group by visited_on