How To Create Auto Generated Primary Key in Oracle Database

For the first time in my life (finally) I decided to make an article about oracle database. This semester is my first time learn about database, and you know, the curriculum in my country is very suck. Until almost at the end of the semester, I just learned about CDM, PDM, DDL, DML, and some basic queries. So, is that all about database? Of course not. How about sequence, trigger, and anything else. Unfortunately, I have to wait next semester to learn about it (it’s really waste of time).

CREATE TABLE HUMAN 
(
  HUMAN_ID VARCHAR2(3) NOT NULL, 

  HUMAN_NAME VARCHAR2(20), 
  CONSTRAINT HUMAN_PK PRIMARY KEY 
  (
    HUMAN_ID 
  )
);

CREATE SEQUENCE HUMAN_PK_SQ
INCREMENT BY 1 
START WITH 1 
MAXVALUE 99 
MINVALUE 1 
CYCLE 
NOCACHE 
ORDER;

CREATE OR REPLACE TRIGGER HUMAN_PK_TG
before insert on HUMAN
for each row 
declare
  max_human_id number;
  cur_id number;
begin
  if inserting then       
    if :NEW."HUMAN_ID" is null then
      -- if HUMAN_ID is not inserted, add new generated ID
      select concat('H', to_char(HUMAN_PK_SQ.nextval, 'FM00')) 
      into :NEW."HUMAN_ID" 
      from dual;
    else
      -- HUMAN_ID is inserted, update the sequence
      select to_number(substr(greatest(max(HUMAN_ID), :NEW."HUMAN_ID"), 2))
      into max_human_id from HUMAN;
      select HUMAN_PK_SQ.nextval into cur_id from dual;
      while cur_id < max_human_id
      loop
        select HUMAN_PK_SQ.nextval into cur_id from dual;
      end loop;
    end if;    
  end if; 
end;

INSERT INTO HUMAN (HUMAN_NAME) VALUES ('Junichi');
INSERT INTO HUMAN (HUMAN_NAME) VALUES ('Baka');
INSERT INTO HUMAN (HUMAN_NAME) VALUES ('Junian');