多项选择题
A.CREATE B.DROP C.DELETE D.USAGE
A.直接执行DROP USER app_r; B.先执行DROP OWNED BY app_r;再执行DROP USER app_r; C.先执行REASSIGN OWNED BY app_rTOsystem;再执行DROP USER app_r; D.直接执行DROP ROLE app_r CASCADE;
A.SQL自动补齐 B.SQL结果排序 C.数据库对象ER图 D.初始化数据库实例