text: create table Ads
  (
      ad_id   int                                   null,
      user_id int                                   null,
      action  enum ('Clicked', 'Viewed', 'Ignored') null
  );
  
  INSERT INTO leetcode_easy.Ads (ad_id, user_id, action) VALUES (1, 1, 'Clicked');
  INSERT INTO leetcode_easy.Ads (ad_id, user_id, action) VALUES (2, 2, 'Clicked');
  INSERT INTO leetcode_easy.Ads (ad_id, user_id, action) VALUES (3, 3, 'Viewed');
  INSERT INTO leetcode_easy.Ads (ad_id, user_id, action) VALUES (5, 5, 'Ignored');
  INSERT INTO leetcode_easy.Ads (ad_id, user_id, action) VALUES (1, 7, 'Ignored');
  INSERT INTO leetcode_easy.Ads (ad_id, user_id, action) VALUES (2, 7, 'Viewed');
  INSERT INTO leetcode_easy.Ads (ad_id, user_id, action) VALUES (3, 5, 'Clicked');
  INSERT INTO leetcode_easy.Ads (ad_id, user_id, action) VALUES (1, 4, 'Viewed');
  INSERT INTO leetcode_easy.Ads (ad_id, user_id, action) VALUES (2, 11, 'Viewed');
  INSERT INTO leetcode_easy.Ads (ad_id, user_id, action) VALUES (1, 2, 'Clicked');