title: How to replace empty string with null value in oracle using nvl function
body: <pre><code>INSERT INTO books(book_id, title, author_last_name, author_first_name, classify)
  VALUES (1000, 'The Picture of Dorian Gray', 'Wilde', 'Oscar', 'fiction');
  INSERT INTO books(book_id, title, author_last_name, author_first_name, classify)
  VALUES (1001, 'Think Big', 'Carson', 'Dr. Ben', 'non - fiction');
  INSERT INTO books(book_id, title, author_last_name, author_first_name, classify)
  VALUES (1003, 'Mathematical Scandals', 'Pappas', ' ', 'non - fiction');
  INSERT INTO books(book_id, title, author_last_name, author_first_name, classify)
  VALUES (1004, 'SQL', 'Harris', 'Andy', 'non - fiction');
  INSERT INTO books(book_id, title, author_last_name, author_first_name, classify)
  VALUES (1010, 'Excel 2016', 'Chan', ' ', 'non - fiction');
  commit;
  </code></pre>
  
  <p>here is the value of my table. in some records i have inserted empty string ''. 
  how to replace that empty string with NULL values using nvl function</p>