Working With Sequences

Objectives

This lesson covers the following objectives:

• List at least three useful characteristics of a sequence
• Write and execute a SQL statement that creates a sequence
• Query the data dictionary using USER_SEQUENCES to confirm a sequence definition
• Apply the rules for using NEXTVAL to generate sequential unique numbers in a table
• List the advantages and disadvantages of caching sequence values
• Name three reasons why gaps can occur in a sequence

Purpose

• Can you image how tedious it would be to have to enter the names of the 30,000 people who enter the London Marathon into a database, while making sure that no one was given the same identification number?
• What if you went to lunch and when you returned, someone else had entered some of the runners’ applications?
• How would you know where to start again?
• Fortunately, SQL has a process for automatically generating unique numbers that eliminates the worry about the details of duplicate numbers.
• The numbering process is handled through a database object called a SEQUENCE.

The Sequence Object

• You already know how to create two kinds of database objects, the TABLE and the VIEW.
• A third database object is the SEQUENCE.
• A SEQUENCE is a shareable object used to automatically generate unique numbers.
• Because it is a shareable object, multiple users can access it.
• Typically, sequences are used to create a primary-key value.
• As you’ll recall, primary keys must be unique for each row. The sequence is generated and incremented (or decremented) by an internal Oracle routine.
• This object is a time-saver for you because it reduces the amount of code you need to write.
• Sequence numbers are stored and generated independently of tables.
• Therefore, the same sequence can be used for multiple tables.
• To create a SEQUENCE:

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

Creating a Sequence

• In the SEQUENCE created for the London Marathon runners, the numbers will increment by 1, starting with the number 1.
• In this case, beginning the sequence with 1 is probably the best starting point.

CREATE SEQUENCE runner_id_seq
INCREMENT BY 1
START WITH 1
MAXVALUE 50000
NOCACHE
NOCYCLE;

• It is a tradition that the best runner in the elite group wears number 1.
• For other situations, such as department IDs and employee IDs, the starting number may be assigned differently.
• Because there will be at least 30,000 runners, the sequence’s maximum value was set well above the expected number of runners.

CREATE SEQUENCE runner_id_seq
INCREMENT BY 1
START WITH 1
MAXVALUE 50000
NOCACHE
NOCYCLE;

• The NOCACHE option prevents values in the SEQUENCE from being cached in memory, which in the event of system failure prevents numbers pre-allocated and held in memory from being lost.

• The NOCYCLE option prevents the numbering from starting over at 1 if the value 50,000 is exceeded.
• Don’t use the CYCLE option if the sequence is used to generate primary-key values unless there is a reliable mechanism that deletes old rows faster than new ones are added.

Confirming Sequences

• To verify that a sequence was created, query the USER_OBJECTS data dictionary.
• To see all of the SEQUENCE settings, query the USER_SEQUENCES data dictionary as shown below.
• List the value names in the SELECT statement as shown below.

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

• If NOCACHE is specified, the last_number column in the query displays the next available sequence number.
• If CACHE is specified, the last_number column displays the next available number in the sequence which has not been cached into memory.

NEXTVAL and CURRVAL Pseudocolumns

• The NEXTVAL pseudocolumn is used to extract successive sequence numbers from a specified sequence.
• You must qualify NEXTVAL with the sequence name.
• When you reference sequence.NEXTVAL, a new sequence number is generated and the current sequence number is placed in CURRVAL.
• The example below inserts a new department in the DEPARTMENTS table.
• It uses the DEPARTMENTS_SEQ sequence for generating a new department number as follows:

INSERT INTO departments
(department_id, department_name, location_id)
VALUES (departments_seq.NEXTVAL, 'Support', 2500);

Suppose now you want to hire employees to staff the new department.
• The INSERT statement to be executed for all new employees can include the following code:

INSERT INTO employees
(employee_id, department_id, ...)
VALUES (employees_seq.NEXTVAL, dept_deptid_seq .CURRVAL, ...);

• Note: The preceding example assumes that a sequence called EMPLOYEES_SEQ has already been created for generating new employee numbers.
• The CURRVAL pseudocolumn in the example below is used to refer to a sequence number that the current user has just generated.
• NEXTVAL must be used to generate a sequence number in the current user’s session before CURRVAL can be referenced.
• You must qualify CURRVAL with the sequence name.
• When sequence.CURRVAL is referenced, the last value generated by that user’s process is returned.

Using a Sequence

• After you create a sequence, it generates sequential numbers for use in your tables. Reference the sequence values by using the NEXTVAL and CURRVAL pseudocolumns.
• You can use NEXTVAL and CURRVAL in the following contexts:
  • The SELECT list of a SELECT statement that is not part of a subquery
  • The SELECT list of a subquery in an INSERT statement
  • The VALUES clause of an INSERT statement
  • The SET clause of an UPDATE statement
• You cannot use NEXTVAL and CURRVAL in the following contexts:
  • The SELECT list of a view
  • A SELECT statement with the DISTINCT keyword
  • A SELECT statement with GROUP BY, HAVING, or ORDER BY clauses
  • A subquery in a SELECT, DELETE, or UPDATE statement
  • The DEFAULT expression in a CREATE TABLE or ALTER TABLE statement
• To continue our London Marathon example, a table was created for the runners:

CREATE TABLE runners
(runner_id NUMBER(6,0) CONSTRAINT runners_id_pk PRIMARY KEY,
first_name VARCHAR2(30),
last_name VARCHAR2(30));

• We then create the sequence that will generate values for the runner_id primary key column.

CREATE SEQUENCE runner_id_seq
INCREMENT BY 1
START WITH 1
MAXVALUE 50000
NOCACHE
NOCYCLE;

• Using the following syntax would allow new participants to be inserted into the runners table.
• The runner’s identification number would be generated by retrieving the NEXTVAL from the sequence.

INSERT INTO runners
(runner_id, first_name, last_name)
VALUES (runner_id_seq.NEXTVAL, 'Joanne', 'Everely');

INSERT INTO runners
(runner_id, first_name, last_name)
VALUES (runner_id_seq.NEXTVAL, 'Adam', 'Curtis');

• To confirm the sequence worked correctly, we query the table:

SELECT runner_id, first_name, last_name
FROM runners;

• To view the current value for the runners_id_seq, CURRVAL is used.
• Note the use of the DUAL table in this example.
• Oracle Application Express will not execute this query, but you should understand how this works.

SELECT runner_id_seq.CURRVAL
FROM dual;

• Cache sequences in memory provide faster access to sequence values.
• The cache is populated the first time you refer to the sequence.
• Each request for the next sequence value is retrieved from the cached sequence.
• After the last sequence value is used, the next request for the sequence pulls another cache of sequences into memory.
• 20 is the default number of sequence numbers cached.

Nonsequential Numbers

• Although sequence generators issue sequential numbers without gaps, this action occurs independently of a database commit or rollback.
• Gaps (nonsequential numbers) can be generated by:
  • Rolling back a statement containing a sequence, the number is lost.
  • A system crash. If the sequence caches values into the memory and the system crashes, those values are lost.
  • The same sequence being used for multiple tables. If you do so, each table can contain gaps in the sequential numbers.

Viewing the Next Value

• If the sequence was created with NOCACHE, it is possible to view the next available sequence value without incrementing it by querying the USER_SEQUENCES table.

SELECT sequence_name, min_value, max_value, last_number AS "Next number"
FROM USER_SEQUENCES
WHERE sequence_name = 'RUNNER_ID_SEQ';

Modifying a Sequence

• As with the other database objects you’ve created, a SEQUENCE can also be changed using the ALTER SEQUENCE statement.
• What if the London Marathon exceeded the 50,000 runner registrations and you needed to add more numbers?
• The sequence could be changed to increase the MAXVALUE without changing the existing number order.

ALTER SEQUENCE runner_id_seq
INCREMENT BY 1
MAXVALUE 999999
NOCACHE
NOCYCLE;

• Some validation is performed when you alter a sequence.
• For example, a new MAXVALUE that is less than the current sequence number cannot be executed.

ALTER SEQUENCE runner_id_seq
INCREMENT BY 1
MAXVALUE 90
NOCACHE
NOCYCLE;

ERROR at line 1:
ORA-04009: MAXVALUE cannot be made to be less than the current value

ALTER SEQUENCE Guidelines

• A few guidelines apply when executing an ALTER SEQUENCE statement.
• They are:
  • You must be the owner or have the ALTER privilege for the sequence in order to modify it.
  • Only future sequence numbers are affected by the ALTER SEQUENCE statement.
  • The START WITH option cannot be changed using ALTER SEQUENCE. The sequence must be dropped and re-created in order to restart the sequence at a different number.

Removing a Sequence

• To remove a sequence from the data dictionary, use the DROP SEQUENCE statement.
• You must be the owner of the sequence or have DROP ANY SEQUENCE privileges to remove it.
• Once removed, the sequence can no longer be referenced.

DROP SEQUENCE runner_id_seq;

Terminology

Key terms used in this lesson included:

• CACHE/ NOCACHE
• CREATE SEQUENCE
• CURRVAL
• CYCLE/ NOCYCLE
• INCREMENT BY
• MAXVALUE
• MINVALUE
• NEXTVAL
• NOMAXVALUE
• NOMINVALUE
• Sequences
• START WITH

Summary

In this lesson, you should have learned how to:

• List at least three useful characteristics of a sequence
• Write and execute a SQL statement that creates a sequence
• Query the data dictionary using USER_SEQUENCES to confirm a sequence definition
• Apply the rules for using NEXTVAL to generate sequential unique numbers in a table
• List the advantages and disadvantages of caching sequence values
• Name three reasons why gaps can occur in a sequence