Oracle 11g: ow to INSERT a row in a table if this row doesn't already exist in same table, in PL/SQL (oracle) : please guide how to INSERT a row in a table if this row doesn't already exist in same table, in PL/SQL (oracle)?
I want something like this.

    insert into note (note_id, user_book_id, course_user_id, book_edition_id, book_id, role_type_id, page_id, book_page_number, xcoord, ycoord,
		                    width, height, share_across_courses, date_created, date_updated, created_by, updated_by, description, share_with_students,text)

    select  note_s.nextval, i_user_book_id, i_course_user_id, book_edition_id, book_id, n.role_type_id, page_id, book_page_number, xcoord, 		    ycoord, width, height, share_across_courses, sysdate, sysdate, i_user_id, i_user_id, description, share_with_students,text

    from  note n inner join course_user cu
    on n.course_user_id = cu.course_user_id
    where     cu.course_id = 23846
    and where not exists (select  note_s.nextval, i_user_book_id, i_course_user_id, book_edition_id, book_id, n.role_type_id, page_id, book_page_number, xcoord, ycoord,
		                width, height, share_across_courses, sysdate, sysdate, i_user_id, i_user_id, description, share_with_students,text

    from  note n inner join course_user cu
    on n.course_user_id = cu.course_user_id
    where     cu.course_id = 23846);
						
That is, in note table if record is already present for a particular course_user_id then do nothing. Otherwise if no entry for that particular course_user_id then insert into note for that course_user_id.
						

But my code is not working.
Pls Note - Here, note_id is PRIMARY KEY in note table and
			 Course_user_id is PRIMARY KEY in course_user table.