title: Sql select query help needed
body: <p>I have the following table structure inside my aanmeldingen table:<br><br></p>
  
  <pre><code>username      varchar(16) NOT NULL
  introduce_van varchar(16) NULL
  </code></pre>
  
  <p>I have the follow 3 rows of data:<br></p>
  
  <pre><code>Username Introduce_van   
  Test1    NULL
  Test2    Test1
  Test3    NULL
  </code></pre>
  
  <p>Now I want to select all username are not in Introduce_van and Introduce_van's value is NULL.</p>
  
  <p>I tried it with this sql:</p>
  
  <pre><code>SELECT username 
  FROM aanmeldingen 
  WHERE username NOT IN
        (SELECT introduce_van  
         FROM aanmeldingen 
         WHERE introduce_van != 'NULL')
  </code></pre>
  
  <p>This query returns:</p>
  
  <pre><code>Test2, Test3
  </code></pre>
  
  <p>So I thought If I would add:</p>
  
  <pre><code>AND introduce_van = 'NULL'
  </code></pre>
  
  <p>It would return just Test3.</p>
  
  <p>But it returns 0 rows.</p>
  
  <p>I just can't figure out how to fix my select query to make sure it only return Test3(Not in Introduce_van and having Introduce_van NULL.</p>
  
  <p>Can someone help me out with this query?</p>