带有 GROUP BY 的 SQL JOIN 语句

Question

The Problem:

Construct the SQL statement to find the number of attendees for every meeting. Display the following columns:

• Count of meeting attendees
• Meeting ID
• Meeting start date and time
• Meeting end date and time

There are 5 tables in this database(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 |
+-----------+------------+

My SQL statement:

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;

The error I get:

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

Please Help me, I am not sure What I am doing wrong. I have not been able to find a solution for this problem

Answer 1

Never use commas in the FROM clause. Always use proper, explicit, standard JOIN syntax.

Don't give you columns aliases that need to be escaped. So, use underscores rather than spaces. This fixes your problem with the weird double quote characters:

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 ;

Answer 2

Based on the error text I would try changing the name of your COUNT() column to just 'Count', since the error message starts just after the first space in your column name

EDIT:

I will also second Gordon Linoff's statement: never do this:

FROM tbla a, tblb b
WHERE a.columna = b.columnb

That's a regular old join, and it should be written as such:

FROM tbla A
JOIN tblb B ON a.columna = b.columnb

Answer 3

There are 2 things that can be changed. First meeting_id column belongs to both table so you have to define which table's column. And then the column naming must have error depending on error description you have provided.

SELECT Count(person_id) AS "Count" ,M.meeting_id,meeting_start,meeting_end    
FROM meeting M 
INNER JOIN person_meeting PM ON M.meeting_id=PM.meeting_id
Group by M.meeting_id,M.meeting_start,M.meeting_end;