title: How to store multiple selected values from <select> tag in a single field
body: <p>I have a select tag with <code>multiple="multiple"</code>. And a user can select more than one value. </p>
  
  <pre><code>&lt;label for="aoi"&gt;Area of Interest:&lt;/label&gt;
  &lt;select id="sel_aoi" name="aoi" multiple="multiple"&gt;
      &lt;option value="hr-executive"&gt;HR Executives&lt;/option&gt;
      &lt;option value="sr-manager"&gt;Sr. Manager&lt;/option&gt;
      &lt;option value="service-advisor"&gt;Service Advisor&lt;/option&gt;
      &lt;option value="production"&gt;Production Engineer&lt;/option&gt;
      &lt;option value="mechanical"&gt;Mechanical Engineer&lt;/option&gt;
  &lt;/select&gt;
  </code></pre>
  
  <p>How I can store these more than one selected values in single field of MySQL database?</p>