09. Sequence in oracle

09. Sequence in oracle

In Oracle, if you want to realize the self-increment operation of a column, you must use the sequence to complete

SQL statement to create sequence

create sequence seq_name
  increment by n  -- 每次增长几
  start with n    -- 从哪个值开始增长
  maxvalue n|nomaxvalue 10^27 or -1  -- 最大值
  minvalue n|no minvalue  -- 最小值
	cycle|nocycle           -- 是否有循环
	cache n|nocache          -- 是否有缓存

Create sequence operation

-- 简单创建
create sequence my_sequence 
increment by 2 
start with 1

how to use

-- 注意,如果创建好序列之后,没有经过任何的使用,那么不能获取当前值
-- 查看序列的当前值
select my_sequence.currval from dual;
-- 提示错误:[72000][8002] ORA-08002: 序列 MY_SEQUENCE.CURRVAL 尚未在此会话中定义
-- 执行查看序列的下一个值
select my_sequence.nextval from dual;
-- 执行完再看当前值
select my_sequence.currval from dual;
-- 执行结果: 1
-- 然后再插入表中的sql中进行使用
insert into emp(empno,ename) values (my_sequence.nextval,'hehe');
select * from emp;