10、（5-6） choose the best answer: Examine the structure of the EMPLOYEES table: There is a parent/child relationship between EMPLOYEE_ID and MANAGER_ID. You want to display the name, joining date, and manager for all the employees. Newly hired employees are yet to be assigned a department or a manager. For them, 'No Manager' should be displayed in the MANAGER column.
Which SQL query gets the required output? A) SELECT e.last_name, e.hire_date, NVL(m.last_name, 'No Manager') Manager FROM employees e RIGHT OUTER JOIN employees m ON (e. manager_id = m.employee_id);
B) SELECT e.last_name, e.hire_date, NVL(m.last_name, 'No Manager') Manager FROM employees e JOIN employees m ON (e.manager_id = m.employee_id);
C) SELECT e.last_name, e.hire_date, NVL(m.last_name, 'No Manager') Manager FROM employees e LEFT OUTER JOIN employees m ON (e.manager_id = m.employee_id);
D) SELECT e.last_name, e.hire_date, NVL(m.last_name, 'No Manager') Manager FROM employees e NATURAL JOIN employees m ON (e.manager_id = m.employee_id);
Answer：C （解析：因为员工 king 是没有经理的，但是也要显示出来，因为是在经理号这边缺少数据，所以这里要用 左外连接。注意左右的区别： SELECT e.last_name, e.hire_date, NVL(m.last_name, 'No Manager') Manager FROM employees e , employees m WHERE e.manager_id = m.employee_id(+); ）
