title: How to store uknown number of job fields
body: <p>I have this job offering system - each offer can be posted for unknown number of work categories (Finance, IT, Services, ...) and can be posted for unknown number of regions (Europe, Asia, America,...). First thing that came into my mind was to create an extra table that would just store <code>region_id, category_id, offer_id</code> but that seems a bit redundant isnt it? So a second thought was to serialize data e.g. as JSON and then store it in DB - but I also want to retrieve those data and join them on table that goes like <code>region_id region_name</code> and <code>category_id category_name</code> and that seems to be performance-hard to do  when it would be stored as JSON isnt it?</p>
  
  <p>So how do I store these uknown number of ids? Can it be effectively done without relations?</p>
  
  <p>Thanks</p>
  
  <p><strong>PS:</strong> I am using PHP&amp;MySQL</p>