drop table invoices;
create table invoices
( invoiceid number
, customerid number
, type varchar2(1)
, amount number
);
insert into invoices(invoiceid, customerid, type, amount)
values(1, 1, 'I', 100);
insert into invoices(invoiceid, customerid, type, amount)
values(2, 1, 'I', 25);
insert into invoices(invoiceid, customerid, type, amount)
values(3, 1, 'I', 15);
insert into invoices(invoiceid, customerid, type, amount)
values(4, 1, 'I', 30);
insert into invoices(invoiceid, customerid, type, amount)
values(5, 1, 'C', -20);
insert into invoices(invoiceid, customerid, type, amount)
values(6, 1, 'C', -30);
insert into invoices(invoiceid, customerid, type, amount)
values(7, 1, 'C', -65);
insert into invoices(invoiceid, customerid, type, amount)
values(8, 2, 'I', 35);
insert into invoices(invoiceid, customerid, type, amount)
values(9, 2, 'I', 50);
insert into invoices(invoiceid, customerid, type, amount)
values(10, 3, 'I', 20);
insert into invoices(invoiceid, customerid, type, amount)
values(11, 3, 'C', -30);
insert into invoices(invoiceid, customerid, type, amount)
values(12, 4, 'C', -50);
select * from invoices