title: SQL string matching
body: <p>I use a string for store the days of the week, something like this:
  MTWTFSS. And if I search for MF (Monday and Friday) then the query must return all the strings that contain MF (for example: MWF, MTWTFS, MF,  and so on).</p>
  
  <p>I don't know how to do this in SQL (MySQL).</p>