Create Sequence – Generador de indentificadores unicos que no bloquea transacciones

Crea un objeto capaz de darnos numeros consecutivos unicos.

CREATE SEQUENCE secuencia
INCREMENT BY n
START WITH n
{MAX VALUE n | NOMAXVALUE}
{MIN VALUE N | NOMINVALUE}
{CYCLE | NOCYCLE}
{CACHE N | NOCACHE}
{ORDER | NOORDER};

En realida es un generador de indentificadores unicos que no bloquea transacciones.
Es muy util para generar primary keys.

DROP TABLE students CASCADE CONSTRAINTS;

CREATE TABLE students (
id                       NUMBER(5) PRIMARY KEY,
first_name       VARCHAR2(20),
last_name        VARCHAR2(20),
major                VARCHAR2(30),
current_credits  NUMBER(3)
);

CREATE SEQUENCE student_sequence
START WITH 10000
INCREMENT BY 1;

INSERT INTO students (id, first_name, last_name, major,
current_credits)
VALUES (student_sequence.NEXTVAL, ‘Scott’, ‘Smith’,
‘Computer Science’, 11);

INSERT INTO students (id, first_name, last_name, major,
current_credits)
VALUES (student_sequence.NEXTVAL, ‘Margaret’, ‘Mason’,
‘History’, 4);

SELECT * FROM students;

Asi obtenemos el siguiente valor:
SELECT student_sequence.NEXTVAL FROM DUAL;

Tambien podemos obtener el valor actual:
SELECT student_sequence.CURRVAL FROM DUAL;