title: Adding If condition to where clause in DQL / Symfony 3
body: <p>I've a doctrine entity with two variables <code>$price</code> &amp; <code>$price_new</code> mapped to mysql table with same column name. 
  <code>$price</code> contains legacy values and <code>$price_new</code> contains new values.
  I want to write a query which checks if <code>$price_new &gt; 0</code> then apply where on <code>$price_new</code> but if <code>$price_new = 0</code>, <code>$price</code> is used in where clause.</p>
  
  <p>Is there any way to implement it in DQL?</p>