title: SQL -- Finding students taking 2 or more classes
body: <p>I had this question in an interview, and I couldn't quite figure it out.  I have three tables: a student table, a classes table and a table linking the other two.  Here is a basic schema for them.</p>
  
  <pre><code>STUDENT
  student_id | student_name
  -----------+-------------
  int        | varchar(30)
  
  CLASS
  class_id | class_name
  ---------+-----------
  int      | varchar(30)
  
  ROSTERS
  student_id | class_id
  -----------+---------
  int        | int
  </code></pre>
  
  <p>The rosters table shows which students are taking which classes.  How do I write a query showing which students are taking 2 or more classes?</p>