title: mysql - find unique users across all sites
body: <p>I have user table which has below data:</p>
  
  <pre><code>user_id      site_unique_id    user_email
  1            1                 steve@gmail.com
  2            2                 steve@gmail.com
  3            3                 steve@gmail.com
  4            1                 mark@gmail.com
  5            3                 mark@gmail.com
  6            1                 jack@gmail.com
  7            2                 jack@gmail.com
  8            3                 jack@gmail.com
  9            1                 smith@gmail.com
  10           2                 smith@gmail.com
  </code></pre>
  
  <p>I want to make mysql query to get user emails that exist in all 3 sites i.e.</p>
  
  <pre><code>email_id           site_unique_id
  steve@gmail.com    1,2,3
  jack@gmail.com     1,2,3
  </code></pre>
  
  <p>in above data mark &amp; jack are not exist in site 2 &amp; 3 respectively those emails not needed in sql result.</p>