title: How to get total unique users as well as grouping by category
body: <p>I have the following SQL table:</p>
  
  <pre><code>user_id | device | num_uses
  ---------------------------
  1       | phone  | 10
  1       | tv     | 5
  2       | phone  | 15
  3       | tv     | 45
  </code></pre>
  
  <p>I want to come up with a query which will return the number of users per device, but also the total number of unique users.</p>
  
  <p>Is it possible to do this with one query, or is my only option to create 2 queries, one for the number of users per device, then another to get the distinct number of users?</p>
  
  <p>The query I have to get the number of users by device is:</p>
  
  <pre><code>select count(user_id), device from usage group by device;
  </code></pre>
  
  <p>which gives me:</p>
  
  <pre><code>count  | device
  ---------------
  2      | phone
  2      | tv
  </code></pre>
  
  <p>Expected result would be:</p>
  
  <pre><code>category_count | device   | total_distinct_users
  ------------------------------------------------
  2              | phone    |  3
  2              | tv       |  3
  </code></pre>