How to use sequences in Oracle

2 years ago
Isabella Edwards
2 minutes

In Oracle databases, a Sequence is an object used to generate unique number sequences. It is commonly used to generate unique primary key values, often in conjunction with auto-incremented primary keys.

To use a Sequence, you first need to create a Sequence object. You can use the CREATE SEQUENCE statement to create a Sequence. For example:

CREATE SEQUENCE seq_name
    START WITH initial_value
    INCREMENT BY increment_value
    MAXVALUE max_value
    MINVALUE min_value
    CYCLE | NOCYCLE
    CACHE cache_size;

seq_name refers to the name of the sequence, initial_value is the starting value, increment_value is the amount added each time, max_value is the highest value, min_value is the lowest value, CYCLE indicates that the sequence will restart from the minimum value after reaching the maximum value, NOCYCLE means that the sequence will stop once the maximum value is reached, and cache_size is the size of the cache.

Once the Sequence has been created, you can use the nextval function to retrieve the next Sequence value, and use the currval function to retrieve the current Sequence value. For example:

SELECT seq_name.nextval FROM dual;
SELECT seq_name.currval FROM dual;

Dual is a unique table used for returning calculation results.

In the case of auto-increment primary keys, you can use the Sequence’s nextval function to obtain the next unique primary key value when inserting data. For example:

INSERT INTO table_name (id, column1, column2)
VALUES (seq_name.nextval, value1, value2);

This will automatically generate a unique primary key value each time data is inserted.

It is important to note that sequences are generated at the database level, not in the application. Therefore, if the same sequence is used in multiple sessions simultaneously, it may result in non-unique generated values. To ensure uniqueness, locking mechanisms can be used, or concurrent access can be limited in the application.

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