Как посчитать количество идентичный пользователей арендателей и арендаторов.

 WITH active_users AS (  
  SELECT DISTINCT user_id AS id 
  FROM Reservations 
  UNION 
  SELECT DISTINCT owner_id 
  FROM Rooms 
  JOIN Reservations 
    ON Reservations.room_id = Rooms.id 
)  
  
SELECT  
  ROUND(100.0 * COUNT(id) / ( 
    SELECT COUNT(id) FROM Users 
    ), 2) AS percent  
FROM active_users;