title: using sql concat() in django for concatenating two columns
body: <p>Let's say I have a table xyz in MySQL which has three columns: col1, col2 and col3 (all varchar). I am using python with Django for my server which uses this model. Now I want to write an expression in Django ORM which should result in this query in MySQL:</p>
  
  <pre><code>SELECT * FROM xyz WHERE col3 != CONCAT(col1, col2)
  </code></pre>
  
  <p>I know Django <code>filter</code>/<code>exclude</code>, but than filters based on fixed values, not columns:</p>
  
  <pre><code>xyz.objects.exclude(col3='something')
  </code></pre>
  
  <p>or</p>
  
  <pre><code>xyz.objects.filter(col3='something')
  </code></pre>
  
  <p>(PS: I know it's a bad table design if one column contains redundant information from other columns)</p>