Get products which are first products that people buy

query.sql

select 
  ProductID,
  sum(case 
    when first_order_datetime is null then 0
    else 1
  end)
from order_lines 
left join (
  select 
    min(Datetime) as first_order_datetime,  
    CustomerID as order_customer_id
  from order_lines
  group by CustomerID
) as first_orders
on Datetime = first_order_datetime 
and CustomerID = order_customer_id
group by ProductID

setup.sql

create table order_lines(
  OrderLineID integer,
  ProductID integer, 
  Datetime datetime,
  CustomerID integer
);
insert into order_lines () values (100, 200, '2017-11-20 00:00:00', 300);
insert into order_lines () values (100, 201, '2017-11-20 00:00:00', 300);
insert into order_lines () values (100, 202, '2017-11-20 00:00:00', 300);

insert into order_lines () values (100, 203, '2017-11-21 00:00:00', 300);
insert into order_lines () values (100, 204, '2017-11-21 00:00:00', 300);

insert into order_lines () values (100, 202, '2017-11-20 00:00:00', 301);
insert into order_lines () values (100, 203, '2017-11-20 00:00:00', 301);

insert into order_lines () values (100, 200, '2017-11-21 00:00:00', 301);
insert into order_lines () values (100, 201, '2017-11-21 00:00:00', 301);
insert into order_lines () values (100, 204, '2017-11-21 00:00:00', 301);

insert into order_lines () values (100, 200, '2017-11-21 00:00:00', 302);
insert into order_lines () values (100, 204, '2017-11-22 00:00:00', 302);
insert into order_lines () values (100, 205, '2017-11-22 00:00:00', 302);