Oracle case when decode and sequence

select grade,decode(grade,1,'a',2,'b','c') from salgrade;
select grade,case when grade=1 then 'a' when grade=2 then 'b' else 'c' end from salgrade;

create sequence empno_seq
maxvalue 99999
start with 15
increment by 1
;

select empno_seq.nextval from dual;