1)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)

　　2)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;

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