11.1.1什么是视图 employees vw employees employee id employee name 投影 employee_id employee position employee name employee age employee position employee address employees vw_employees_hr employee_id employee id employee name 投 employee name employee position employee_position employee age employee age employee_address employee_address vw_employees employee_id employee_name employee_position 投影 employees employee_id employee_name employee_position employee_age employee_address vw_employees_hr employee_id employee_name employee_position employee_age employee_address 投影 employees employee_id employee_name employee_position employee_age employee_address