单项选择题 Examine the structures of the EMPLOYEES and TAX tables. You need to find the percentage tax applicable for each employee. Which SQL statement would you use?（）

单项选择题 You need to modify the STUDENTS table to add a primary key on the STUDENT_ID column. The table is currently empty.Which statement accomplishes this task?（）

单项选择题 You are the DBA for an academic database. You need to create a role that allows a group of users to modify existing rows in the STUDENT_GRADES table.Which set of statements accomplishes this?（）