บทบาทของพนักงานคืออะไรไม่จำเป็นต้องใช้โต๊ะทั้งหมด
What are the staff roles of the staff whoIt is not neccessary to use all the tables.
 Project_Staff:  (role_code [ VARCHAR ] date_from [ VARCHAR ] date_to [ VARCHAR) ]
SELECT role_code FROM Project_Staff WHERE date_from > '2003-04-19 15:06:20' AND date_to < '2016-03-15 00:33:18'