title: MySQL query: ORDER BY date plus other record
body: <p>I have a table with: </p>
  
  <pre><code>last_billed DATE DEFAULT "0000-00-00"
  bill_interval varchar(10) DEFAULT "Monthly" (other values are: Quarterly, Half-yearly, Yearly)
  </code></pre>
  
  <p>I run a simple query, but it needs to: </p>
  
  <pre><code>ORDER BY (last_billed + bill_interval)
  </code></pre>
  
  <p>This should give a date which is 1 month, 3 months, 6 months or 12 months further then the last_billed date, depending on the value of bill_interval.
  Now I can change bill_interval to anything if need be, if it would make the query possible.</p>
  
  <p>I have no idea where to start with this. So any feedback is, as always, appreciated.</p>