mysql - 使用 GROUP BY 的 SQL JOIN 语句
问题描述
问题:
构造 SQL 语句以查找每次会议的参加者人数。显示以下列:
- 会议参加人数
- 会议编号
- 会议开始日期和时间
- 会议结束日期和时间
该数据库中有 5 个表(person、building、room、meeting、person_meeting
+-----------+------------+------------+
| person_id | first_name | last_name |
+-----------+------------+------------+
| 1 | Tom | Hanks |
| 2 | Anne | Hathaway |
| 3 | Tom | Cruise |
| 4 | Meryl | Streep |
| 5 | Chris | Pratt |
| 6 | Halle | Berry |
| 7 | Robert | De Niro |
| 8 | Julia | Roberts |
| 9 | Denzel | Washington |
| 10 | Melissa | McCarthy |
+-----------+------------+------------+
+-------------+----------------------+
| building_id | building_name |
+-------------+----------------------+
| 1 | Headquarters |
| 2 | Main Street Buidling |
+-------------+----------------------+
+---------+-------------+-------------+----------+
| room_id | room_number | building_id | capacity |
+---------+-------------+-------------+----------+
| 1 | 100 | 1 | 5 |
| 2 | 200 | 1 | 4 |
| 3 | 300 | 1 | 10 |
| 4 | 10 | 2 | 4 |
| 5 | 20 | 2 | 4 |
+---------+-------------+-------------+----------+
+------------+---------+---------------------+---------------------+
| meeting_id | room_id | meeting_start | meeting_end |
+------------+---------+---------------------+---------------------+
| 1 | 1 | 2016-12-25 09:00:00 | 2016-12-25 10:00:00 |
| 2 | 1 | 2016-12-25 10:00:00 | 2016-12-25 12:00:00 |
| 3 | 1 | 2016-12-25 11:00:00 | 2016-12-25 12:00:00 |
| 4 | 2 | 2016-12-25 09:00:00 | 2016-12-25 10:00:00 |
| 5 | 4 | 2016-12-25 09:00:00 | 2016-12-25 10:00:00 |
| 6 | 5 | 2016-12-25 14:00:00 | 2016-12-25 16:00:00 |
+------------+---------+---------------------+---------------------+
+-----------+------------+
| person_id | meeting_id |
+-----------+------------+
| 1 | 1 |
| 10 | 1 |
| 1 | 2 |
| 2 | 2 |
| 3 | 2 |
| 4 | 2 |
| 5 | 2 |
| 6 | 2 |
| 7 | 2 |
| 8 | 2 |
| 9 | 3 |
| 10 | 3 |
| 1 | 4 |
| 2 | 4 |
| 8 | 5 |
| 9 | 5 |
| 1 | 6 |
| 2 | 6 |
| 3 | 6 |
+-----------+------------+
我的 SQL 语句:
SELECT Count(person_id) AS “Count of meeting attendees” ,meeting_id,meeting_start,meeting_end
FROM meeting M ,person_meeting PM
WHERE M. meeting_id=PM. meeting_id
Group by PM.meeting_id,M.meeting_start,M.meeting_end;
我得到的错误:
ERROR 1064 (42000): You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near 'of meeting attendees” ,meeting_id,meeting_start,meet
ing_end
FROM meeting M ' at line 1
请帮助我,我不确定我做错了什么。我无法找到解决此问题的方法
解决方案
切勿在FROM
子句中使用逗号。 始终使用正确、明确、标准的JOIN
语法。
不要给你需要转义的列别名。所以,使用下划线而不是空格。这解决了奇怪的双引号字符的问题:
SELECT COUNT(pm.person_id) AS num_attendees,
m.meeting_id, m.meeting_start, m.meeting_end
FROM meeting m JOIN
person_meeting PM
ON m.meeting_id = pm.meeting_id
GROUP BY m.meeting_id, m.meeting_start, m.meeting_end ;
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