In Oracle PL/SQL, a sequence is a database object that generates a unique series of numeric values in ascending or descending order. It is often used to generate primary key values for tables in a database. Sequences are particularly useful in scenarios where unique, sequential identifiers are required, such as when creating primary key values for tables.
Here’s an overview of how sequences work in Oracle PL/SQL:
Creating a Sequence
To create a sequence, you use the CREATE SEQUENCE statement. Here’s a simple example:
CREATE SEQUENCE my_sequence START WITH 1 INCREMENT BY 1 NOCACHE NOCYCLE;
In this example:
my_sequence is the name of the sequence.
START WITH 1 specifies the initial value of the sequence.
INCREMENT BY 1 indicates that the sequence should increment by 1 each time a new value is generated.
NOCACHE specifies that the sequence should not be cached (all values are generated and stored in the database).
NOCYCLE means that the sequence will not cycle back to its starting value after reaching its maximum or minimum value.
Using a Sequence:
Once a sequence is created, you can use the NEXTVAL or CURRVAL pseudocolumns to retrieve the next or current value of the sequence, respectively.
INSERT INTO my_table (id, name) VALUES (my_sequence.NEXTVAL, 'John Doe');
In this example, my_sequence.NEXTVAL generates the next value from the sequence, which is then used as the primary key for the my_table table.
Altering a Sequence:
You can alter the properties of a sequence using the ALTER SEQUENCE statement. For example, you can change the increment value, cache size, or restart the sequence.
ALTER SEQUENCE my_sequence INCREMENT BY 2;
This alters the sequence to increment by 2 instead of 1.
Dropping a Sequence:
If a sequence is no longer needed, you can use the DROP SEQUENCE statement to remove it from the database.
DROP SEQUENCE my_sequence;
Be cautious when dropping a sequence, as it may affect any tables that use it for generating primary key values.
Sequences in Oracle PL/SQL provide an efficient and reliable way to generate unique identifiers, especially in multi-user environments where generating unique values can be challenging without a centralized mechanism.