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WITH operations AS ( -- Операции выдачи SELECT staff_id, DATE(rental_date) AS operation_date, 'rental' AS operation_type FROM rental WHERE YEAR(rental_date) = 2005 AND MONTH(rental_date) = 5 UNION ALL -- Операции возврата SELECT staff_id, DATE(return_date) AS operation_date, 'return' AS operation_type FROM rental WHERE return_date IS NOT NULL AND YEAR(return_date) = 2005 AND MONTH(return_date) = 5 ), working_days AS ( SELECT Count(distinct operation_date) as w_days From operations ) select s.staff_id, s.first_name, s.last_name, count(*) as average_operation_count From staff s Join operations o ON o.staff_id = s.staff_id cross JOIN working_days wd group by s.staff_id, s.first_name, s.last_name -- wd.w_days