text: CREATE TABLE books (
  	isbn_13 serial PRIMARY KEY,
  	book_name VARCHAR(50),
  	author VARCHAR(50)
  );
  
  CREATE TABLE tags (
  	isbn_13 INTEGER REFERENCES books(isbn_13),
  	tag_name VARHCAR(50) UNIQUE
  	
  	CONSTRAINT tags_pk PRIMARY KEY (isbn_13, tag_name) 
  );
  
  
  -- Harry Potter : "Young Adult", "Fiction", "Fantasy", "Magical"
  -- Lord oF the Rings : "Fiction"