title: Check if column value exists in another column in SQL
body: <p>I need a SQL query that compares two values and returns an ID.</p>
  
  <p>I have this table:</p>
  
  <pre><code>ID  Calling_ID  Called_ID
  1   27          10
  2   15          20
  3   80          90
  4   90          88
  5   60          30
  6   88          40
  7   15          60
  8   30          40
  9   27          95
  10  40          30
  </code></pre>
  
  <p>How do I check if each value in the <code>Calling_ID</code> column exists in the <code>Called_ID</code> column and then return the ID? The above data would return 88, 30, 40.</p>