SQL Challenges-1: Show running quantiry for each unit type of item

Create table  sale (product_name varchar(25),unit_type varchar(5),sale_date date,sale_qty integer);

insert into sale  values ('Munchos','P','2018-05-15',20);
insert into sale  values ('Boyer Chocolate','P','2018-04-27', 30);
insert into sale  values ('CocaCola','L','2018-04-10', 25);
insert into sale  values ('Fruit Cakes','P','2018-07-12', 30);
insert into sale  values ('CocaCola','L','2018-07-07', 50);
insert into sale  values ('Fanta','L','2018-01-27', 70);
insert into sale  values ('Chex Mix','P','2018-09-17', 40);
insert into sale  values ('Jaffa Cakes','P','2018-06-25', 40);
insert into sale  values ('Pom-Bear','P','2018-02-11', 30);
insert into sale  values ('Twix Chocolate','P','2018-12-24', 50);
insert into sale  values ('Limca','L','2018-03-15', 50);
insert into sale  values ('Mirinda','L','2018-02-05', 40);



select
    s1.unit_type,s1.sale_date,
    sum(s2.sale_qty) as "running unit"
from sale as s1
join sale as s2
on s1.sale_date >= s2.sale_date and s1.unit_type = s2.unit_type
group by s1.unit_type,s1.sale_date
order by unit_type,sale_date;