oracle自增列创建方法收藏
Oracle没有自增字段这样的功能,但是通过(trigger)和序列(sequence)可以实现。
先建一个测试表了:
create table userlogin
(
id number(6) not null,
name varchar2(30) not null primary key
)
tablespace users
/
第一步:创建SEQUENCE
create sequence userlogin_seq increment by 1 start with 1 minvalue 1 maxvalue 9999999999999 nocache order;
第二步:创建一个基于该表的before insert 触发器,在触发器中使用刚创建的SEQUENCE
create or replace trigger userlogin_trigger
before insert on userlogin
for each row
begin
userlogin_seq.nextval into:new.id from sys.dual ;
end;
/
第三步:在userlogin表中测试
写个insert语句,插入一条记录,看ID字段自增了没,自增则OK啦。
oracle没有ORACLE自增字段这样的功能,但是通过触发器(trigger)和序列(sequence)可以实现。
先建一个测试表了:
create table userlogin
(
id number(6) not null,
name varchar2(30) not null primary key
)
tablespace users
/
第一步:创建SEQUENCE
create sequence userlogin_seq increment by 1 start with 1 minvalue 1 maxvalue 999999999 nocache order;
第二步:创建一个基于该表的before insert 触发器,在触发器中使用刚创建的SEQUENCE
create or replace trigger userlogin_trigger
before insert on userlogin
for each row
begin
select userlogin_seq.nextval into:new.id from sys.dual ;
end;
/
第三步:在userlogin表中测试
写个insert语句,插入一条记录,看ID字段自增了没,自增则OK啦。
Oracle序列知识:
在oracle中sequence就是所谓的序列号,每次取的时候它会自动增加,一般用在需要按序列号排序的地方。
1、Create Sequence
你首先要有CREATE SEQUENCE或者CREATE ANY SEQUENCE权限,
CREATE SEQUENCE emp_sequence
INCREMENT BY 1 -- 每次加几个
START WITH 1 -- 从1开始计数
NOMAXVALUE -- 不设置最大值
NOCYCLE -- 一直累加,不循环
CACHE 10;
一旦定义了emp_sequence,你就可以用CURRVAL,NEXTVAL
CURRVAL=返回 sequence的当前值
NEXTVAL=增加sequence的值,然后返回 sequence 值
比如:
emp_sequence.CURRVAL
emp_sequence.NEXTVAL
可以使用sequence的地方:
- 不包含子查询、snahot、VIEW的 SELECT 语句
- INSERT语句的子查询中
- NSERT语句的VALUES中
- UPDATE 的 SET中
可以看如下例子:
INSERT INTO emp VALUES
(empseq.nextval, 'LEWIS', 'CLERK',7902, SYSDATE, 1200, NULL, 20);
SELECT empseq.currval FROM DUAL;
但是要注意的是:
- 第一次NEXTVAL返回的是初始值;随后的NEXTVAL会自动增加你定义的INCREMENT BY值,然后返回增加后的值。CURRVAL 总是返回当前SEQUENCE的值,但是在第一次NEXTVAL初始化之后才能使用CURRVAL,否则会出错。一次NEXTVAL会增加一次SEQUENCE的值,所以如果你在同一个语句里面使用多个NEXTVAL,其值就是不一样的。明白?
- 如果指定CACHE值,ORACLE就可以预先在内存里面放置一些sequence,这样存取的快些。cache里面的取完后,oracle自动再取一组到cache。 使用cache或许会跳号, 比如突然不正常down掉(shutdown abort),cache中的sequence就会丢失. 所以可以在create sequence的时候用nocache防止这种情况。
2、Alter Sequence
你或者是该sequence的owner,或者有ALTER ANY SEQUENCE 权限才能改动sequence. 可以alter除start至以外的所有sequence参数.如果想要改变start值,必须 drop sequence 再 re-create .
Alter sequence 的例子
ALTER SEQUENCE emp_sequence
INCREMENT BY 10
MAXVALUE 10000
CYCLE -- 到10000后从头开始
NOCACHE ;
影响Sequence的初始化参数:
SEQUENCE_CACHE_ENTRIES =设置能同时被cache的sequence数目。
可以很简单的Drop Sequence
DROP SEQUENCE order_seq;

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