title: ClickHouse, find ads with first event "click"
body: <p>I have a table with columns [datetime, ad_id, event, platform], where</p>
  
  <p>ad_id - id of advertisement</p>
  
  <p>event - {'view', 'click'}</p>
  
  <p>platform - {'web', 'android', 'ios'}</p>
  
  <p>I want to find ad_ids which were clicked before viewed.</p>
  
  <p>How can i do it?</p>