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