title: If else on WHERE clause
body: <p>I've this query:</p>
  
  <pre><code>SELECT  `id` ,  `naam` 
  FROM  `klanten` 
  WHERE (
  `email` LIKE  '%@domain.nl%'
  OR  `email2` LIKE  '%@domain.nl%'
  )
  </code></pre>
  
  <p>But I want to do something like this:</p>
  
  <pre><code>SELECT  `id` ,  `naam` 
  FROM  `klanten` 
  WHERE IF(`email` &gt; 0,
  `email` LIKE  '%@domain.nl%'
  ,  `email2` LIKE  '%@domain.nl%'
  )
  </code></pre>
  
  <p>How to check if email exist? I want to use email and if this field is empty I want to use email2. How do I accomplish this?</p>