【071题库】最新OCP-071考试题库解析-第31题
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071题库-第31题、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(+);
)
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