Creating sequences is an important aspect of Oracle SQL. Sequences are database objects from which multiple users may generate unique integers. It is possible to define certain aspects like the first value to start with, the increment size, and the maximum limit, among others. The numbers generated by a sequence can be used for several purposes such as generating unique identifiers, primary keys, control numbers, and many more.
What is a Sequence in Oracle SQL?
A sequence in Oracle SQL is a database object that lets you generate unique integer values. This becomes notably useful when you need to create a unique primary key value, or any unique value within a database. It is efficient and convenient as it prevents concurrency issues, reduces network traffic, and avoids the overhead costs of maintaining a transaction-based assignment approach.
CREATE SEQUENCE seq_name
START WITH initial_value
INCREMENT BY increment_value
MAXVALUE maximum_value
Creating a Sequence
To create a sequence in Oracle, you can use the CREATE SEQUENCE statement. The sequence generator begins with an initial value, increments by a specific value, and never exceeds a maximum value. If no parameters are defined, default values are used. The sequence can be increasing or decreasing, depending on your preference.
CREATE SEQUENCE test_seq
START WITH 1
INCREMENT BY 1
MAXVALUE 10000
The above code will create a sequence named ‘test_seq’. The sequence will start at 1, increment by 1 for each next value, and will not exceed the maximum value of 10000.
Utilizing the Created Sequence
To utilize the created sequence, you call the NEXTVAL function on the sequence object which increments the sequence and returns the new value. The CURRVAL function can be used to retrieve the current value of the sequence without incrementing it. However, CURRVAL cannot be referred before NEXTVAL has been called in a session.
INSERT INTO test_table (id, data) VALUES (test_seq.NEXTVAL, ‘some data’);
Modifying and Removing a Sequence
Existing sequences can be altered using the ALTER SEQUENCE command where one can change parameters like INCREMENT, MAXVALUE, MINVALUE etc. If a sequence needs to be removed, the DROP SEQUENCE command is used.
Alter sequence:
ALTER SEQUENCE test_seq
INCREMENT BY 2
MAXVALUE 50000
Drop sequence:
DROP SEQUENCE test_seq
In conclusion, sequences in Oracle SQL offer a powerful tool for generating unique numbers, primary keys, and other values in a multi-user environment. They are easy to use, configurable, and efficient due to their standalone nature not requiring any transactional controls.
