Creating a trigger that will update an attribute - sql

Question

Question: write a trigger that will update the invoice subtotal each time the line table is updated (a new row inserted, updated or deleted). Include the SQL statements used to test the insert, update and delete.
I'm finding it difficult to properly understand triggers and I don't know why. I understand the basic concept of it (at least I think I do) but I can't seem to understand how to answer my question. The following code is my attempt at answering the above question:
create or replace trigger update_subtotal
after insert or update or delete
on invoice
for each row
begin
insert into line ('inv_number', 'line_number', 'p_code', 'line_units', 'line_price')
values ('1009', '3', '12345-6t', '1', '123.45');
end;
select * from line;
After running this code I ended up with these errors:
Errors: TRIGGER UPDATE_SUBTOTAL
Line/Col: 3/1 PL/SQL: SQL Statement ignored
Line/Col: 3/19 PL/SQL: ORA-00928: missing SELECT keyword
Line/Col: 17/1 PLS-00103: Encountered the symbol "SELECT"
I'm using Oracle Live.
In short: help.

Answer 1

You seem to have the concept backwards. The invoice table needs to be updated when line changes -- so line needs the trigger and the change to invoice is an update. That would be something like this:
create or replace trigger trg_line_update_subtotal
after insert or update or delete
on line
for each row
begin
update invoice i
set total = coalesce(i.total, 0) +
coalesce(:new.line_Units * :new.line_price, 0) -
coalesce(:old.line_Units * :old.line_price, 0)
where i.inv_number = coalesce(:new.inv_number, :old.inv_number);
end;