OCP-1Z0-051-V9.02-126题

126. View the Exhibit and examine the data in the PROJ_TASK_DETAILS table.

The PROJ_TASK_DETAILS table stores information about tasks involved in a project and the relation

between them.

The BASED_ON column indicates dependencies between tasks. Some tasks do not depend on the

completion of any other tasks.

You need to generate a report showing all task IDs, the corresponding task ID they are dependent on, and

the name of the employee in charge of the task it depends on.

Which query would give the required result? 

A. SELECT p.task_id, p.based_on, d.task_in_charge

FROM proj_task_details p JOIN proj_task_details d

ON (p.based_on = d.task_id);

B. SELECT p.task_id, p.based_on, d.task_in_charge

FROM proj_task_details p LEFT OUTER JOIN proj_task_details d

ON (p.based_on = d.task_id);

C. SELECT p.task_id, p.based_on, d.task_in_charge

FROM proj_task_details p FULL OUTER JOIN proj_task_details d

ON (p.based_on = d.task_id);

D. SELECT p.task_id, p.based_on, d.task_in_charge

FROM proj_task_details p JOIN proj_task_details d

ON (p.task_id = d.task_id);

Answer: B

答案解析：

参考：http://blog.csdn.net/rlhua/article/details/12877591

显示所有任务的ID，依赖对应任务的ID，和它依赖任务负责的员工姓名

A为只显示有依赖任务的task_id

B是LEFT OUTER JOIN也就是左连表的所有task_id全部显示出来，如果右边表没有匹配的信息，则显示空