选择题：

　　1)Which statement shows the maximum salary paid in each job category of each department?_______

　　A、 select dept_id， job_cat，max(salary) from employees where salary > max(salary);

　　B、 select dept_id， job_cat，max(salary) from employees group by dept_id，job_cat;

　　C、 select dept_id， job_cat，max(salary) from employees;

　　D、 select dept_id， job_cat，max(salary) from employees group by dept_id;

　　E、 select dept_id， job_cat，max(salary) from employees group by dept_id，job_cat，salary;

　　2)description of the students table:

　　sid_id number

　　start_date date

　　end_date date

　　which two function are valid on the start_date column?_________。

　　A、 sum(start_date)

　　B、 avg(start_date)

　　C、 count(start_date)

　　D、 avg(start_date，end_date)

　　E、 min(start_date)

　　F、 maximum(start_date)

　　3)for which two constraints does the oracle server implicitly create a unique index?______。