A、 not null

　　B、 primary

　　C、 foreign key

　　D、 check

　　E、 unique

　　4)in a select statement that includes a where clause，where is the group by clause placed in the select statement?______。

　　A、 immediately after the select clause

　　B、 before the where clause

　　C、 before the from clause

　　D、 after the order by clause

　　E、 after the where clause

　　5)in a select statement that includes a where clause，where is the order by clause placed in the select statement?______。

　　A、immediately after the select clause

　　B、before the where clause

　　C、after all clause

　　D、after the where clause

　　E、before the from clause

　　6)evaluate there two sql statements______。

　　Select last_name，salary from employees order by salary;

　　Select last_name，salary from employees order by 2 asc;

　　A。the same result B。different result C。the second statement returns a syntax error