title: Can't UNION on primary key
body: <p>So these are 2 queries:</p>
  
  <pre><code>SELECT * FROM CourseGrade; 
  SELECT * FROM LetterGrades;
  </code></pre>
  
  <p><img src="https://i.stack.imgur.com/TsmEQ.png" alt="enter image description here"></p>
  
  <p>When I do this UNION to see which LetterGradeId has which LetterGrade:</p>
  
  <pre><code>SELECT * 
      FROM CourseGrade
      INNER JOIN LetterGrades
      ON CourseGrade.LetterGradeId=LetterGrades.LetterGradeId; 
  </code></pre>
  
  <p>I'm getting an error saying </p>
  
  <pre><code>Invalid column name 'LetterGradeId'.
  </code></pre>
  
  <p>Can anyone tell me where am I going wrong?</p>