title: ORA-04098: trigger is invalid and failed re-validation error
body: <p>Why this procedure can't run? It compiles correctly but when i try to insert something it shows me this error: <strong>ORA-04098: trigger 'SAMPLEDATA.BI_STUDENTS' is invalid and failed re-validation</strong>.</p>
  
  <pre><code>create or replace PROCEDURE insert_new_student (
    pName   NVARCHAR2, 
    pPhone NVARCHAR2,
    pReg DATE,      
    pBirth DATE
  )
  AS
    -- declare variables.
    v_student_id NUMBER;
  BEGIN
    -- add a record.
    INSERT INTO STUDENTS (STUDENT_ID, NAME, PHONENUMBER, REGISTRATIONDATE, BIRTHDAY)
    VALUES (student_id_seq.nextval, pName, pPhone, pReg, pBirth)
    RETURNING STUDENT_ID INTO v_student_id;
  END insert_new_student;
  </code></pre>
  
  <p>//</p>
  
  <pre><code>BEGIN
    insert_new_student( 'A', '012345', SYSDATE, '01/26/2017');
  END
  </code></pre>
  
  <p>//</p>
  
  <pre><code>CREATE OR REPLACE EDITIONABLE TRIGGER  "BI_STUDENTS" 
    before insert on "STUDENTS"               
    for each row  
  begin   
    if :NEW."STUDENT_ID" is null then 
      select "STUDENTS_SEQ".nextval into :NEW."STUDENT_ID" from sys.dual; 
    end if; 
  end; 
  </code></pre>