title: SQL: return string describing user category based on a field
body: <p>The idea is to get in the result a new column describing the user "category". This category could for example be "under18","normal","over65" based on user age.</p>
  
  <p>That is, for each user in my table I want to get as result its ID and this description in a column called "category".</p>
  
  <p>How would you proceed? I need this because the data will later go to a JSON object and to a ExtJS grid for visualization.</p>