currval &nextval的差异理解

 --currval/nextval的区别  
   
 select * from 订单
 delete from 订单 where 订单编号=40
 --nextval
INSERT INTO 订单(订单编号,单价)    --nextval每执行一次,会在下列插入一行,序号会依次增加
VALUES      (订单_订单编号_seq.nextval, 
           4500); 
 --currval
INSERT INTO 订单(订单编号,单价)    --currval每执行一次,会在下列插入一行,序号不会增加
VALUES      (订单_订单编号_seq.currval, 
           4500);      
--每次必须先有nextval运行一次之后才可以试验currval