如何计算另一个表中的行以影响另一个表

Question

I have the following tables and have been able to get a result of classes that start after a particular time on a certain date. What I want to do is somehow only show classes that have less than 10 bookings associated.

I have created this sql query to get the classes that a particular user is able to book, but I want to hide the classes where there are already 10 people already booked in the class in question. Ie the class is full if there are 10 or more associated bookings so I dont want to show these classes.

Any help would be much appreciated.

SELECT DISTINCT b.name
              , a.time 
           FROM class a   
           JOIN class_detail b 
             ON a.class_id = b.id   
           JOIN branch c 
             ON a.branch_id = c.id 
          WHERE c.level <= ( SELECT d.level 
                               FROM client d 
                              WHERE d.facebook_id = 'xxxxxx'
                           )
            AND a.date = '2016-08-17' 
            AND a.time >= '13.00.00';

BOOKINGS
+-----------+-------------+------+-----+---------+-------+
| Field     | Type        | Null | Key | Default | Extra |
+-----------+-------------+------+-----+---------+-------+
| id        | bigint(20)  | NO   | PRI | NULL    |       |
| CLIENT_ID | int(11)     | NO   |     | NULL    |       |
| CLASS_ID  | int(11)     | NO   |     | NULL    |       |
| STATUS    | varchar(10) | NO   |     | NULL    |       |
+-----------+-------------+------+-----+---------+-------+

mysql> show fields from BRANCH;
+---------------------+-------------+------+-----+---------+----------------+
| Field               | Type        | Null | Key | Default | Extra          |
+---------------------+-------------+------+-----+---------+----------------+
| id                  | int(10)     | NO   | PRI | NULL    | auto_increment |
| NAME                | char(50)    | NO   |     | NULL    |                |
| CONTACT_NO          | char(50)    | YES  |     | NULL    |                |
| MAP_IMG_PATH        | char(200)   | YES  |     | NULL    |                |
| ADDRESS             | char(200)   | YES  |     | NULL    |                |
| LEVEL               | int(2)      | NO   |     | NULL    |                |
| LOCATION            | int(10)     | YES  |     | NULL    |                |
| SECTOR_NAME         | varchar(45) | YES  |     | NULL    |                |
| SECTOR_MAP_IMG_PATH | char(200)   | YES  |     | NULL    |                |
+---------------------+-------------+------+-----+---------+----------------+



mysql> show fields from CLIENT;
+--------------+-------------+------+-----+---------+-------+
| Field        | Type        | Null | Key | Default | Extra |
+--------------+-------------+------+-----+---------+-------+
| id           | int(10)     | NO   | PRI | NULL    |       |
| NAME         | char(50)    | NO   |     | NULL    |       |
| DOB          | int(8)      | NO   |     | NULL    |       |
| LOCAL_BRANCH | int(10)     | YES  |     | NULL    |       |
| FACEBOOK_ID  | char(50)    | NO   |     | NULL    |       |
| START_DATE   | int(8)      | NO   |     | NULL    |       |
| EMAIL        | char(50)    | YES  |     | NULL    |       |
| PIN          | int(4)      | YES  |     | NULL    |       |
| END_DATE     | int(8)      | NO   |     | NULL    |       |
| LEVEL        | int(2)      | YES  |     | NULL    |       |
| TEL          | varchar(20) | YES  |     | NULL    |       |
+--------------+-------------+------+-----+---------+-------+



mysql> show fields from CLASS_DETAIL;
+--------------+--------------+------+-----+---------+-------+
| Field        | Type         | Null | Key | Default | Extra |
+--------------+--------------+------+-----+---------+-------+
| id           | int(10)      | NO   | PRI | NULL    |       |
| NAME         | char(50)     | NO   |     | NULL    |       |
| DESCRIPTION  | char(200)    | NO   |     | NULL    |       |
| CATEGORY     | varchar(4)   | YES  |     | NULL    |       |
| ACHIEVE_TYPE | char(200)    | YES  |     | NULL    |       |
| IMG_M        | varchar(200) | YES  |     | NULL    |       |
| IMG_F        | varchar(200) | YES  |     | NULL    |       |
+--------------+--------------+------+-----+---------+-------+



mysql> show fields from CLASS;
+-----------+---------+------+-----+---------+----------------+
| Field     | Type    | Null | Key | Default | Extra          |
+-----------+---------+------+-----+---------+----------------+
| id        | int(11) | NO   | PRI | NULL    | auto_increment |
| CLASS_ID  | int(10) | YES  |     | NULL    |                |
| BRANCH_ID | int(10) | NO   |     | NULL    |                |
| DURATION  | int(3)  | YES  |     | NULL    |                |
| DATE      | date    | NO   |     | NULL    |                |
| TIME      | time    | NO   |     | NULL    |                |
| STATUS    | char(1) | NO   |     | NULL    |                |
+-----------+---------+------+-----+---------+----------------+
7 rows in set (0.11 sec)

Answer 1

Here is how you can do this without making changes to your tables.

SELECT DISTINCT b.name
              , a.time 
           FROM class a 
           Inner join (SELECT class_id, count(clientid) 
                       FROM bookings 
                       GROUP BY class_id
                       HAVING count(clientid) < 10) as openClasses on        
                a.class_id = openClasses.class_id
           JOIN class_detail b 
             ON a.class_id = b.id   
           JOIN branch c 
             ON a.branch_id = c.id 
          WHERE c.level <= ( SELECT d.level 
                               FROM client d 
                              WHERE d.facebook_id = 'xxxxxx'
                           )
            AND a.date = '2016-08-17' 
            AND a.time >= '13.00.00';

This query uses a derived table, which I called "openClasses". The point of the table is to get the class_ids of the classes that have less than 10 bookings. This table is then inner joined to restrict results to only this set of classes.

It may be that you have to add a where clause into this derived table to restrict to only certain statuses.