title: Compare table content with external file- SQL
body: <p>I have an Excel file with users IDs, called <code>users Excel</code>.  </p>
  
  <p>In the Database I have a <code>users</code> table: this table holds the data from the users excel file and from another file (administrators file).</p>
  
  <p>I would like to check if I have a user in the <code>users Excel</code> which does not exist in the table.</p>
  
  <p>Should I build a virtual table? how?</p>