Sequence: SEQUENCE

First, the sequence of presentation

Oracle database object is a sequence, the main role is used to generate a unique value. Objects can be found in the sequence after sequence data dictionary is created, so the sequence can be shared by multiple objects.

Second, create a sequence

Sequence created using the CREATE SEQUENCE syntax:

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

NCREMENT BY: used to define a sequence of steps, if omitted, it defaults to 1, and if negative, the sequence of the representative value is decreasing.
START WITH: initial value (i.e., generating a first value) is defined sequences, the default is one.
MAXVALUE: maximum value defined sequence can be generated. NOMAXVALUE is the default option, representing no maximum time, for incrementing the sequence, the system can generate a maximum power of 10 to 27; for a descending sequence, the maximum value is -1.
MINVALUE: define the minimum sequence can be generated. NOMINVALUE is the default, not representative of the minimum, then, for a descending sequence, the system can generate the minimum value is a negative power of 10 to 26; for a descending sequence, the minimum is 1.
CYCLE and NOCYCLE: sequence generator when the value indicates whether the cycle limit is reached. If the cycle, when the sequence reaches the maximum value is incremented, the minimum cycle; For a descending sequence, at the minimum value, maximum value to the cycle. If not recycled, the limit is reached, continue to generate new value error occurs.
CACHE: the size of the memory block storing the sequence defined, the default is 20. NOCACHE not expressed sequence buffer memory. Sequence of memory buffer, can improve the performance sequence.

E.g:

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

Third, the query sequence

Once the sequence is created, the code sequence was created textual data dictionary, the data can be seen in user_objects dictionary, such as:

SELECT object_name,object_id,object_type  FROM user_objects WHERE object_name = 'INVOICE_SEQ';

Save the sequence detailed information in user_sequences table:

SELECT sequence_name, min_value, max_value, increment_by, last_number  FROM user_sequences;

Fourth, the use of sequence

And dummy column NEXTVAL CURRVAL

NEXTVAL: Returns the next available sequence value, which is returned each time a unique reference value, the actual true for different users. When using sequence.NEXTVAL, a new sequence number is generated and the current sequence number is placed CURRVAL.
CURRVAL: you get the current sequence value. On the use of CURRVAL before the first use NEXTVAL, it will complain.

Use as follows:

SELECT invoice_seq.CURRVAL,invoice_seq.NEXTVAL FROM DUAL;

INSERT INTO invoice  (invoice_id, vendor_id, invoice_number, invoice_total  )
      VALUES (invoice_seq.NEXTVAL, 10, 'INV' || invoice_seq.CURRVAL, 100  );

CURRVAL NEXTVAL and may be used in the following context:

SELECT statement is not part of the sub-query list field.
List SELECT INSERT statement neutron query.
INSERT statement in the VALUES clause.
UPDATE statement SET clause.

Can not be used in the context of the following NEXTVAL and CURRVAL:

SELECT list view.
DISTINCT with SELECT statement.
With GROUP BY, HAVING or ORDER BY clause of the SELECT statement.
Clause in SELECT, DELETE, or UPDATE statement.
DEFAULT expression in a CREATE TABLE or ALTER TABLE statement.

Also note, ROLLBACK sequence value can not roll back.

Fifth, modify the sequence

Such as:

ALTER SEQUENCE invoice_seq     INCREMENT BY 2 MAXVALUE 10   NOCACHE  NOCYCLE;

When modifying sequence, we have the following limitations:

The sequence can start value table.
It is not greater than the current minimum value.
Not less than the current maximum value.
Sequence revised rules will not affect the value of the previous sequence, only the future sequence value will be affected.
The user must have the permission of ALTER SEQUENCE.

Six deleted sequence

DROP SEQUENCE invoice_seq;