title: SQL Join did not work (zero records return)?
body: <p>I found this question from a tutorial,</p>
  
  <pre><code>Emp( eid: integer, ename: string, age: integer, salary: real)
  Works( eid: integer, did: string, pct-time: integer)
  Dept( did: string, budget: real, managerid: integer)
  </code></pre>
  
  <p>As that question Employee can work many departments. Work table used to map employees and departments. The pct_time field of the Works relation shows the percentage of time that a given employee works in a given department.</p>
  
  <p><strong>Then it ask to print names and ages of each employee who works in both the Hardware and Software department.</strong></p>
  
  <p>For that I have written following SQL but it did not work? can someone please explain the issue. </p>
  
  <pre><code>SELECT Emp.ename, Emp.age
  FROM Emp
  JOIN Works ON Works.eid = Emp.eid
  WHERE Works.did = 'Hardware' AND Works.did = 'Software'
  </code></pre>