title: Oracle SQL SELECT does not work
body: <p>I have this SELECT in Oracle:</p>
  
  <pre><code>SELECT
       po.id, po.name
  FROM 
       poffice po
  WHERE 
  po.id not in 
  (
      select distinct 
          poffice_id 
      from pdistrict
  );
  </code></pre>
  
  <p>which does not return any values.
  BUT the next code returns some records. </p>
  
  <pre><code>SELECT
     po.id, po.name
  FROM
      poffice po
      LEFT JOIN pdistrict pd 
      ON po.ID = pd.poffice_id
  WHERE pd.ID IS NULL;
  </code></pre>
  
  <p>Could anybody explain WHY? Where I'm wrong?</p>