SQL: How to Find Holes in Sequences

create table tbl (id number);
create table missing (id number);

insert into tbl values (1);
insert into tbl values (2);
insert into tbl values (3);
insert into tbl values (4);
insert into tbl values (5);
insert into tbl values (10);
insert into tbl values (11);
insert into tbl values (15);
commit;

declare
	i number;
	idmax number;
	idmin number;
	curr number;
begin
	select max(id) into idmax from tbl where id>0;
	select min(id) into idmin from tbl;
	for i in idmin..idmax
	loop
		select count(*) into curr from tbl where id=i;
		if curr =0 then
			insert into missing values (i);
			commit;
		end if;
		if mod(i,100) = 0 then
			commit;
		end if;
	end loop;
	commit;
end;
/

select * from tbl;

        ID
----------
         1
         2
         3
         4
         5
        10
        11
        15

8 rows selected.

select * from missing;

        ID
----------
         6
         7
         8
         9
        12
        13
        14

7 rows selected.