title: Should primary key in all table set as foreign key in other table?
body: <p>I am doing my database assignment with the business system, airline reservation system.</p>
  
  <p>And I found that all primary keys in my ERD table are a foreign key in other tables except the <code>psg_seat_no</code> in <code>ticket_detail</code> table. Is that normal?</p>
  
  <p>Here is my ERD:</p>
  
  <p><a href="https://i.stack.imgur.com/IMdhr.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/IMdhr.png" alt="enter image description here"></a></p>