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HomeDatabaseMysql TutorialOracle 中创建序列

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。

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、snapshot、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, 比如数据库突然不正常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。 

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