Oracle 생 성 시퀀스 트리거

declare 
      num   number; 
begin
-- prompt dropping sequence 
      num := 0;
      select count(1) into num from user_sequences where sequence_name = 'COMMON_MYTASK_SEQUENCE'; 
      if num > 0 then   
         execute immediate 'DROP SEQUENCE  COMMON_MYTASK_SEQUENCE';   
      end if;
-- prompt dropping trigger      
      num := 0;
      select count(1) into num from user_triggers where trigger_name = 'COMMON_MYTASK_TG'; 
      if num > 0 then   
         execute immediate 'DROP TRIGGER  COMMON_MYTASK_TG';   
      end if;
-- prompt Dropping 
      num := 0;
      select count(1) into num from user_tables where TABLE_NAME = 'T_COMMON_MYTASK';
      if   num=1   then 
          execute immediate 'drop table T_COMMON_MYTASK'; 
      end   if; 


end;
/


--     
CREATE TABLE T_COMMON_MYTASK(
	id                NUMBER               NOT NULL,
	process_definition_id varchar2(100)	not null,--    id
	request_staff_id	number		not null,--   id
	approver_id		number		not null,--   id
	process_type		number		not null,--    ：  ；  ；....
	crt_time	date,--    
	
   CONSTRAINT PK_T_COMMON_MYTASK PRIMARY KEY ("ID")
);


--create sequence
create sequence COMMON_MYTASK_SEQUENCE
start with 1
increment by 1
nomaxvalue
nocycle
cache 20;
-- create trigger
CREATE OR REPLACE TRIGGER COMMON_MYTASK_TG
  BEFORE INSERT ON T_COMMON_MYTASK
  FOR EACH ROW
  WHEN (new.id is null)
begin
  select COMMON_MYTASK_SEQUENCE.nextval into :new.id from dual;
end COMMON_MYTASK_TG;
/