title: Taking Unique Records From Two Tables
body: <p>i am stuck at a silly problem. And it has to be one of the most cliche solution. </p>
  
  <p><strong>Table</strong> student_selection:</p>
  
  <pre><code>Columns
  =======
  student_id
  subject_id
  faculty_id
  </code></pre>
  
  <p><strong>Another table</strong>: sub_group</p>
  
  <pre><code>Columns
  =======
  subject_id
  sub_group
  </code></pre>
  
  <p>They are Joint on subject_id.</p>
  
  <p>I want to find those subject_id from table sub_group where, the subject_id is not present in the student_selection table.</p>
  
  <p>Eg:</p>
  
  <pre><code>sub_group(subject_id)
  2
  3
  4
  
  student_selection(subject_id)
  2
  3
  2
  3
  2
  3
  </code></pre>
  
  <p><strong>Output</strong></p>
  
  <pre><code>4
  </code></pre>