SQL query to find the number of customers who shopped for 3 consecutive days in month of January 2020

Question

I have below table called orders which has customer id and their order date (Note: there can be multiple orders from same customer on a single day)

create table orders (Id char, order_dt date)

insert into orders values
('A','1/1/2020'),
('B','1/1/2020'),
('C','1/1/2020'),
('D','1/1/2020'),
('A','1/1/2020'),
('B','1/1/2020'),
('A','2/1/2020'),
('B','2/1/2020'),
('C','2/1/2020'),
('B','2/1/2020'),
('A','3/1/2020'),
('B','3/1/2020')

I'm trying to write an SQL query to find the number of customers who shopped for 3 consecutive days in month of January 2020

Based on above order values, the output should be: 2

I referred other similar questions but still wasn't able to come the exact solution

Answer 1

Here is my solution which works fine even there are many orders of one customer in one day;

Some scripts to build test environment:

create table orders (Id varchar2(1), order_dt date);

insert into orders values('A',to_date('01/01/2020','dd/mm/yyyy'));
insert into orders values('B',to_date('01/01/2020','dd/mm/yyyy'));
insert into orders values('C',to_date('01/01/2020','dd/mm/yyyy'));
insert into orders values('D',to_date('01/01/2020','dd/mm/yyyy'));
insert into orders values('A',to_date('01/01/2020','dd/mm/yyyy'));
insert into orders values('B',to_date('01/01/2020','dd/mm/yyyy'));
insert into orders values('A',to_date('02/01/2020','dd/mm/yyyy'));
insert into orders values('B',to_date('02/01/2020','dd/mm/yyyy'));
insert into orders values('C',to_date('02/01/2020','dd/mm/yyyy'));
insert into orders values('B',to_date('02/01/2020','dd/mm/yyyy'));
insert into orders values('A',to_date('03/01/2020','dd/mm/yyyy'));
insert into orders values('B',to_date('03/01/2020','dd/mm/yyyy'));


 select distinct id,  count_days from (
    select id,
           order_dt,
           count(*) over(partition by id order by order_dt range between 1  preceding  and 1 following  )  count_days
    from orders group by  id, order_dt
)
where count_days = 3;

--  Insert for test more days than 3 consecutive

insert into orders values('A',to_date('04/01/2020','dd/mm/yyyy'));