title: difference of with and without single quote
body: <p>I have seen people defined Alias name with and without quotes, but I don't understand the different between them. When and why should we use quotes, and when and why don't we use quotes?</p>
  
  <pre><code>convert(char(20) , usr_name) 'Name'; 
  convert(char(20) , usr_name) "Name"; 
  convert(char(20) , usr_name) Name;
  </code></pre>