为什么我的查询返回“空集（0.00秒）”？

Question

求解运行的UPDATE建筑物SET building_name =“ Main Street Building” WHERE building_id = 2; 从那里，任务3查询将返回正确的结果。

任务一：

查询：

SELECT first_name, last_name, building_name, room.room_id, meeting_start,meeting_end
FROM meeting, person, person_meeting,room, building
WHERE room.room_id=meeting.room_id
AND meeting.meeting_id=person_meeting.meeting_id
AND person.person_id=person_meeting.person_id
AND room.building_id=building.building_id
AND person.first_name='Tom'
AND person.last_name='Hanks';

任务二：

查询：

SELECT first_name, last_name, building_name, room.room_id, meeting_start,meeting_end
FROM meeting, person, person_meeting,room, building
WHERE room.room_id=meeting.room_id
AND meeting.meeting_id=person_meeting.meeting_id
AND person.person_id=person_meeting.person_id
AND room.building_id=building.building_id
AND meeting.meeting_id=2;

任务三：

查询：

SELECT first_name, last_name, building_name, room.room_id,meeting.meeting_id, meeting_start, meeting_end
FROM meeting, person, person_meeting,room, building
WHERE room.room_id=meeting.room_id
AND meeting.meeting_id=person_meeting.meeting_id
AND person.person_id=person_meeting.person_id
AND room.building_id=building.building_id
AND building_name='Main Street Building';

任务四：

查询：

SELECT count(person_id) 'Count of meeting attendees', meeting.meeting_id,meeting_start,meeting_end
FROM meeting, person_meeting
WHERE meeting.meeting_id=person_meeting.meeting_id
GROUP BY meeting.meeting_id;

任务五：

查询：

SELECT first_name, last_name, meeting.meeting_id, meeting_start, meeting_end
FROM meeting INNER JOIN person_meeting ON meeting.meeting_id=person_meeting.meeting_id
INNER JOIN person ON person.person_id=person_meeting.person_id
AND meeting_start<'2016-12-25 12.00.00';

任务1,2,4,5和5都能完美运行。如果所有人都需要任务提示，请告诉我。

Answer 1

注意：我尚未对此进行测试。

我根据我们最近的互动更改了查询。

这是您的原始照片

SELECT first_name, last_name, building_name, room.room_id,meeting.meeting_id, meeting_start, meeting_end    
FROM meeting, person, person_meeting,room, building    
WHERE room.room_id=meeting.room_id    
    AND meeting.meeting_id=person_meeting.meeting_id    
    AND person.person_id=person_meeting.person_id    
    AND room.building_id=building.building_id    
    AND building_name='Main Street Building';

这是我尝试构建的方式

根据文档的第2页和第3页，您提供了https://snhu.brightspace.com/d2l/lor/viewer/viewFile.d2lfile/76437/9523,2/，我将执行以下操作：

首先，我将获得有关建筑物的信息。

SELECT building_id
FROM building
WHERE building_name like  '%Main Street Building%';

注意：我假设建筑物名称正确。但是，以防万一，我在名称前后使用通配符％

接下来，我将获得有关房间的信息：

SELECT b.building_name, r.room_id
FROM room r
  LEFT JOIN building b
    ON r.building_id = b.building_id
WHERE b.building_name like  '%Main Street Building%';

在此之后，我将获得有关会议的信息：

SELECT b.building_name, r.room_id, m.meeting_id, m.meeting_start, m.meeting_end
FROM room r
  LEFT JOIN building b
    ON r.building_id = b.building_id
  LEFT JOIN meeting m
    ON r.room_id = m.room_id
WHERE b.building_name like  '%Main Street Building%';

最后，我将获取将参加该会议的人员的ID信息并显示其姓名：

SELECT b.building_name, r.room_id, m.meeting_id, m.meeting_start, m.meeting_end, p.first_name, p.last_name
FROM room r
  LEFT JOIN building b
    ON r.building_id = b.building_id
  LEFT JOIN meeting m
    ON r.room_id = m.room_id
  LEFT JOIN persom_meeting pm
    ON m.meeting_id = pm.meeting_id
  LEFT JOIN person p
    ON pm.person_id = p.person_id
WHERE b.building_name like  '%Main Street Building%';

如果这些查询不起作用，我建议您确保表之间存在关系。

这意味着表之间应该有building_id匹配，room_id匹配，meeting_id匹配和person_id匹配。另外，由于建筑物名称区分大小写，因此我会检查建筑物名称的拼写是否正确。

我使用LEFT JOIN的原因是，我可以显示先前表（所有行）的所有信息以及ID与外键ID匹配的记录。如此处所述：http://www.acarlstein.com/?p=4168

此外，在http://www.acarlstein.com/?p=4194中，我将展示两个左（外）连接如何工作。

但是，在您的情况下，它更像是一条正在构建的链，因此它与图不匹配，但是您也许可以理解我的意思。