title: SQL database structure with two changing properties
body: <p>Let's assume I am building the backend of a university management software.
  I have a users table with the following columns:</p>
  
  <pre><code>id
  name
  birthday
  last_english_grade
  last_it_grade
  </code></pre>
  
  <p>profs table columns:</p>
  
  <pre><code>id
  name
  birthday
  </code></pre>
  
  <p>I'd like to have a third table with which I can determine all professors teaching a student.
  So I'd like to assign multiple teachers to each student.
  Those Professors may change any time.
  New students may be added any time too.</p>
  
  <p>What's the best way to achieve this?</p>