Chapter 11-Creating Other Schema Objects ---Sequence

Sequences

A sequence:

Can automatically generate unique numbers

Is a shareable object

Can be used to create a primary key value

Replace application code

Speeds up the efficiency of accessing sequence values when cached in memory.

CREATE SEQUENCE Statement Syntax

Define a sequence to generate sequential numbers automatically:

CREATE SEQUENCE sequence
[INCREMENT BY n]
[START WIT n]
[{MAXVALUE n | NOMAXVALUE}]
[{MINVALUE n | NOMINVALUE}]
[{CYCLE | NOCYCLE}]
[{CACHE n | NOCACHE}]

Creating a Sequence

Create a sequence named DEPT_DEPTID_SEQ to be used for the primary key of the DEPARTMENTS table,and Do not use the CYCLE option

CREATE SEQUENCE dept_deptid_seq
INCREMENT BY 10
START WITH 120
MAXVALUE 9999
NOCACHE
NOCYCLE;

NEXTVAL and CURRVAL Pseudocolumns

NEXTVAL returns the next available sequence value.It returns a unique value every time it is referenced,even for different users.

CURRVAL obtains the current sequence value.

NEXTVAL must be issued for that sequence before CURRVAL contains a value.

使用SEQUENCE

如果首次创建SEQUENCE dept_deptid_seq，执行SELECT dept_deptid_seq.CURRVAL FROM DUAL;会报错

SQL> select dept_deptid_seq.currval from dual;
select dept_deptid_seq.currval from dual
*
ERROR at line 1:
ORA-08002: sequence DEPT_DEPTID_SEQ.CURRVAL is not yet defined in this session

必须先执行SELECT dept_deptid_seq.NEXTVAL FROM DUAL;然后再执行SELECT dept_deptid_seq.CURRVAL FROM DUAL;

Caching Sequence Values

Caching sequence values in memory gives faster access to those values

Gaps in sequence values can occur when:

-A rollback occurs

-The system crashes

-A sequence is used in another table

Modifying a Sequence

Change the increment value,maximum value,minimum value,cycle option,or cache option,but you can not modify the start with keyword

ALTER SEQUENCE dept_deptid_seq
INCREMENT BY 20
MAXIMUM 999999
NOCACHE
NOCYCLE;

Guidelines for Modifying a Sequence

You must be the owner or have the ALTER privilege for the sequence

Only future sequence numbers are affected

The sequence must be dropped and re-created to restart the sequence at a different number

Some validation is performed

Drop Sequence

To remove a sequence ,use the DROP SEQUENCE statement

DROP SEQUENCE dept_deptid_seq;