title: Database problem--student registering for the same class twice..how to stop it Access 20007
body: <p>I am making a mock database and I have a <code>student</code> table, <code>student_class</code> linking table, and a <code>class</code> table.  How can I make it so that a student cannot be registered for the same class twice?</p>
  
  <p>Example: StudentID 21 and classID 34 can be entered X number of times in the linking table. </p>
  
  <p>The tables are designed 3NF, too.</p>