title: why use 'NA' = with the possibility of returning a group of values in SAS?
body: <p>I have a quick question about the following piece of code. Why can we use <code>'NA' =</code>  for the subquery ? I mean, the subquery might return a group of values, not a single one, right? Could anyone tell me the reason? Many thanks for your time and attention. </p>
  
  <pre><code>proc sql;
     select lastname, first name
     from sasuser.staffmaster
     where 'NA' = 
        (select jobcategory
         from sasuser.supervisors 
         where staffmaster.empid = supervisors.empid);
  quit;
  </code></pre>
  
  <p>Thanks again.</p>