Oracle中，当需要建立一个自增字段时，需要用到sequence。sequence也可以在mysql中使用，但是有些差别，日后再补充，先把Oracle中sequence的基本使用总结一下，方便日后查阅。
1、创建sequence：
create sequence SEQ_ON_USER minvalue 1 maxvalue 999999999999999999999999999 start with 1 increment by 1 nocache;
说明：
minvalue：序列最小值
maxvalue/nomaxvalue：序列最大值/没有最大值
start with 1：序列从1开始
increment by 1：每次增加1
cache/nocache：nocache不缓存。cache缓存。开启缓存，效率高，只是如果数据库宕机了，缓存丢失，会出现序列跳号情况。
2、查看已有sequence：
select * from user_sequences; select * from user_sequences;
3、删除指定sequence：
DROP SEQUENCE SEQ_ON_USER； DROP SEQUENCE SEQ_ON_USER；
4、查看指定sequence的当前值：
两种方式：
select last_number from user_sequences wheresequence_name='SEQ_ON_USER'; select last_number from user_sequences wheresequence_name='SEQ_ON_USER';
select SEQ_ON_USER.nextval from sys.dual; select SEQ_ON_USER.nextval from sys.dual;
5、创建触发器使用sequence设置主键自动插入。
create or replace trigger "SEQ_ON_USER_GENERATOR" before insert on databasename1.T_USER for each row declare mid number, begin select SEQ_ON_USER.nextval into mid from dual; :new.id:=mid; end create trigger SEQ_ON_USER_Trigger before insert on T_USER for each row begin select SEQ_ON_USER.nextval into :new.id from dual; end SEQ_ON_USER_Trigger; create or replace trigger "SEQ_ON_USER_GENERATOR" before insert on databasename1.T_USER for each row declare mid number, begin select SEQ_ON_USER.nextval into mid from dual; :new.id:=mid; end
create trigger SEQ_ON_USER_Trigger
before insert on T_USER for each row begin select SEQ_ON_USER.nextval into :new.id from dual; end SEQ_ON_USER_Trigger;
6、代码中使用sequence.nextval插入主键值。
