转--Oracle数据库中序列(SEQUENCE)的用法详解

拼搏现实的明天。 2022-06-01 13:20 246阅读 0赞

在Oracle数据库中，序列的用途是生成表的主键值，可以在插入语句中引用，也可以通过查询检查当前值，或使序列增至下一个值。本文我们主要介绍了序列的用法，希望能够对您有所帮助。

AD：

**在Oracle数据库中，什么是序列呢？**它的用途是什么？序列(SEQUENCE)其实是序列号生成器，可以为表中的行自动生成序列号，产生一组等间隔的数值(类型为数字)。其主要的用途是生成表的主键值，可以在插入语句中引用，也可以通过查询检查当前值，或使序列增至下一个值。

创建序列需要CREATE SEQUENCE系统权限。序列的创建语法如下：

CREATE SEQUENCE 序列名 \[INCREMENT BY n\] \[START WITH n\] \[\{MAXVALUE/ MINVALUE n|NOMAXVALUE\}\] \[\] \[\{CACHE n|NOCACHE\}\];

INCREMENT BY 用于定义序列的步长，如果省略，则默认为1，如果出现负值，则代表序列的值是按照此步长递减的。

START WITH 定义序列的初始值(即产生的第一个值)，默认为1。

MAXVALUE 定义序列生成器能产生的最大值。选项NOMAXVALUE是默认选项，代表没有最大值定义，这时对于递增序列，系统能够产生的最大值是10的27次方;对于递减序列，最大值是-1。

MINVALUE定义序列生成器能产生的最小值。选项NOMAXVALUE是默认选项，代表没有最小值定义，这时对于递减序列，系统能够产生的最小值是?10的26次方;对于递增序列，最小值是1。

CYCLE和NOCYCLE 表示当序列生成器的值达到限制值后是否循环。CYCLE代表循环，NOCYCLE代表不循环。如果循环，则当递增序列达到最大值时，循环到最小值;对于递减序列达到最小值时，循环到最大值。如果不循环，达到限制值后，继续产生新值就会发生错误。

CACHE(缓冲)定义存放序列的内存块的大小，默认为20。NOCACHE表示不对序列进行内存缓冲。对序列进行内存缓冲，可以改善序列的性能。

删除序列的语法是：

DROP SEQUENCE 序列名;

其中：

删除序列的人应该是序列的创建者或拥有DROP ANY SEQUENCE系统权限的用户。序列一旦删除就不能被引用了。

序列的某些部分也可以在使用中进行修改，但不能修改SATRT WITH选项。对序列的修改只影响随后产生的序号，已经产生的序号不变。

修改序列的语法如下：

**创建和删除序列**

例1：创建序列：

CREATE SEQUENCE ABC INCREMENT BY 1 START WITH 10 MAXVALUE 9999999 NOCYCLE NOCACHE;

执行结果：

序列已创建。

步骤2：删除序列：

DROP SEQUENCE ABC;

执行结果：

序列已丢弃。

说明：以上创建的序列名为ABC，是递增序列，增量为1，初始值为10。该序列不循环，不使用内存。没有定义最小值，默认最小值为1，最大值为9 999 999。

**序列的使用**

如果已经创建了序列，怎样才能引用序列呢?方法是使用CURRVAL和NEXTVAL来引用序列的值。

在编号的过程中，产生间隙的原因多种多样。如果一个存储过程从一个序列中挑选某个数字，定为本地变量，但是从来不用它，这个数字就丢失了。它将不能再返回到原序列中，结果就造成数值序列中存在一个间隙。关系型数据库模型中不必担心这一点。但是有时候人们在意这一点，这些人想知道是哪些数字丢失了。

调用NEXTVAL将生成序列中的下一个序列号，调用时要指出序列名，即用以下方式调用:

序列名.NEXTVAL

CURRVAL用于产生序列的当前值，无论调用多少次都不会产生序列的下一个值。如果序列还没有通过调用NEXTVAL产生过序列的下一个值，先引用CURRVAL没有意义。调用CURRVAL的方法同上，要指出序列名，即用以下方式调用:

序列名.CURRVAL.

产生序列的值。

步骤1：产生序列的第一个值：

SELECT ABC.NEXTVAL FROM DUAL;

执行结果：

NEXTVAL ------------------ 10

步骤2：产生序列的下一个值：

SELECT ABC.NEXTVAL FROM DUAL;

执行结果：

NEXTVAL ------------------- 11

产生序列的当前值：

SELECT ABC.CURRVAL FROM DUAL;

执行结果：

CURRVAL -------------------- 11

**说明：**第一次调用NEXTVAL产生序列的初始值，根据定义知道初始值为10。第二次调用产生11，因为序列的步长为1。调用CURRVAL，显示当前值11，不产生新值。Oracle的解析函数为检查间隙提供了一种要快捷得多的方法。它们使你在使用完整的、面向集合的SQL处理的同时，仍然能够看到下一个行(LEAD)或者前一行(LAG)的数值。

实例：

1、创建表：

2、创建序列：

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      1 
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      4 
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    <div style="width:1070px; margin:0px auto; border:0px!important; bottom:auto!important; float:none!important; left:auto!important; line-height:1.1em!important; outline:0px!important; overflow:visible!important; position:relative!important; right:auto!important; top:auto!important; vertical-align:baseline!important; min-height:auto!important; padding:0px!important; background:none!important"> 
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      <code style="font-family:Monaco,Menlo,Consolas,'Courier New',monospace; border:0px!important; bottom:auto!important; float:none!important; left:auto!important; line-height:1.1em!important; outline:0px!important; overflow:visible!important; position:static!important; right:auto!important; top:auto!important; vertical-align:baseline!important; width:auto!important; font-weight:700!important; min-height:auto!important; margin:0px!important; padding:0px!important; color:rgb(255,120,0)!important; background:none!important">Create</code>&nbsp; 
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     </div> 
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     </div> 
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     </div> 
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     </div> 
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     </div> 
     <div style="border:0px!important; bottom:auto!important; float:none!important; left:auto!important; line-height:1.1em!important; outline:0px!important; overflow:visible!important; position:static!important; right:auto!important; top:auto!important; vertical-align:baseline!important; width:auto!important; min-height:auto!important; margin:0px!important; padding:0px 1em!important; background:none!important"> 
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     </div> 
    </div> </td> 
  </tr> 
 </tbody> 
</table>

3、插入数据：

[![4034970a304e251fb0945da6a186c9177f3e5308.jpg][]][4034970a304e251fb0945da6a186c9177f3e5308.jpg 1]

4、再插入一条数据：

[![279759ee3d6d55fb11c2cf786b224f4a20a4dd6c.jpg][]][279759ee3d6d55fb11c2cf786b224f4a20a4dd6c.jpg 1]

[4034970a304e251fb0945da6a186c9177f3e5308.jpg]: /images/20220601/adca28dbf1494e46be86e39bf85ad4f4.png
[4034970a304e251fb0945da6a186c9177f3e5308.jpg 1]: https://gss0.baidu.com/-vo3dSag_xI4khGko9WTAnF6hhy/zhidao/pic/item/4034970a304e251fb0945da6a186c9177f3e5308.jpg
[279759ee3d6d55fb11c2cf786b224f4a20a4dd6c.jpg]: /images/20220601/31d6ac54bc3d4d10878d387839a74493.png
[279759ee3d6d55fb11c2cf786b224f4a20a4dd6c.jpg 1]: https://gss0.baidu.com/-Po3dSag_xI4khGko9WTAnF6hhy/zhidao/pic/item/279759ee3d6d55fb11c2cf786b224f4a20a4dd6c.jpg