Oracle 11g Release 2

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CREATE TABLE Student (
    Student_ID number(3) CONSTRAINT Student_ID_pk PRIMARY KEY,
    First_Name VARCHAR2(14) CONSTRAINT First_name_NN  NOT NULL,
    Last_Name VARCHAR(14) CONSTRAINT Last_name_NN NOT NULL
    );
CREATE SEQUENCE STUDENT_ID_SQ 
                INCREMENT BY 111
                START WITH 111
                NOMAXVALUE 
                NOCYCLE;
INSERT INTO Student (Student_ID,First_Name,Last_Name)
        VALUES (STUDENT_ID_SQ.NEXTVAL, 'Jhon','Smith');
1 rows affected

CREATE OR REPLACE TRIGGER TRG_STUDENT_ID 
BEFORE INSERT ON STUDENT
FOR EACH ROW 
BEGIN 
    IF :NEW.STUDENT_ID  IS NULL THEN
    SELECT STUDENT_ID_SQ.NEXTVAL INTO :NEW.STUDENT_ID FROM DUAL;
END IF;
END;
/
INSERT INTO Student (First_Name,Last_Name) VALUES ('John','Smith');
1 rows affected

SELECT * from student;


STUDENT_ID
FIRST_NAME
LAST_NAME


111
Jhon
Smith

222
John
Smith