emp(eno, ename, salary, mgr)，其中mgr是员工领导，设计约束：要求领导的工资不能低于他的下属平均工资。（注意，这是一个递归查询）-emp (eno, ename, salary, mgr), which is the employee mgr leadership, design constraints: the requirement that lead not be lower than the wages of the average wage of his subordinates. (Note that this is a recursive query)