Oracle创建自增列的两种方式

Oracle含有序列的事务进行回滚，序列不会回到事务前状态。

-- 方法一: 序列
-- Created on 2023/3/10 by 112691 
declare 
  -- Local variables here
  v_cnt number;
begin
  -- Test statements here
  select count(*) into v_cnt from user_tables where table_name = upper('user_info');
  if v_cnt>0 then
    execute immediate 'drop table user_info';
    dbms_output.put_line('该表已删除');
  else
    execute immediate 'create table user_info(
                      id number(10) not null,
                      username varchar(20) not null,
                      constraints PK_USERINFO primary key(id)
                      )';
    dbms_output.put_line('该表已创建成功');
  end if;
end;

create sequence USER_INFO_SEQ
start with 1
minvalue 1
maxvalue 999999
increment by 1
-- cache: 申请序列时Oracle将序列提前生成cache x先存入内存。在发送大量的申请序列请求时，会优先去内存中取预存序列。但如果数据库重启时，预存内存中的序列值会被丢失，再次启动时候，预存重上次内存中最大序列号+1开始cache。	
cache 20; 

insert into user_info values(user_info_seq.nextval,'username1');
insert into user_info values(user_info_seq.nextval,'username2');

-- 方法二: 序列+触发器
create or replace trigger TIG_USER_INFO_INSERT
  before insert
  on user_info
  for each row
declare
  -- local variables here
begin
  select user_info_seq.nextval into :new.id from dual;
end TIG_USER_INFO_INSERT;

insert into user_info(username) values('username3');
commit;

drop table user_info ;
drop sequence USER_INFO_SEQ;

参考博客：
https://blog.csdn.net/qq_45554909/article/details/116912915
https://blog.csdn.net/fn0723/article/details/81104861