What are sequences in Oracle databases, and how can they be used for generating unique values?

In Oracle databases, a sequence is a database object that generates a series of unique values. A sequence is often used to provide a unique primary key value for a table.

To create a sequence, you can use the CREATE SEQUENCE statement. Here is an example:

CREATE SEQUENCE myseq
START WITH 1
INCREMENT BY 1;

This creates a sequence named ‘myseq‘ that starts at 1 and increments by 1 for each value generated.

To use the sequence to generate a unique value, you can use the NEXTVAL function. Here is an example:

INSERT INTO mytable (id, name)
VALUES (myseq.NEXTVAL, 'John');

This inserts a new row into ‘mytable‘ with a unique ID value generated by the ‘myseq‘ sequence.

Sequences can also be customized further by specifying minvalue, maxvalue, cycle/no cycle options, cache/no cache option, etc. Moreover, different sequences can be created in different schemas, each with its own characteristics.

In summary, sequences are useful for generating unique values and ensuring that there are no duplicate values in a table’s primary key column.