title: What is the difference between SELECT * and SELECT value
body: <p>What's the difference between</p>
  
  <pre><code>    $query = "SELECT email FROM users WHERE username='$username' ";
      $result = mysqli_query($db, $query);
      $row = mysqli_fetch_array($result);
      $email = $row['email'];
      $_SESSION['email'] = $email;
  </code></pre>
  
  <p>and</p>
  
  <pre><code>    $query = "SELECT * FROM users WHERE username='$username' ";
      $result = mysqli_query($db, $query);
      $row = mysqli_fetch_array($result);
      $email = $row['email'];
      $_SESSION['email'] = $email;
  </code></pre>
  
  <p>They both return the same value, is there any more effective way to do that?</p>