如何在多个表中选择值？

Question

我有以下数据库结构：

car
+----------+------------+----------------+-----------------+
| car_id   | car_name   | car_model_id   | car_factory_id  |
+----------+------------+----------------+-----------------+

car_model
+---------------+----------------+
| car_model_id  | car_model_name |
+---------------+----------------+

factory
+-------------+---------------+-------------------+
| factory_id  | factory_name  | factory_location  |
+-------------+---------------+-------------------+

factory_location
+----------------------+------------------------+
| factory_location_id  | factory_location_name  |
+----------------------+------------------------+

我想：

• 列出所有工厂
• 计算每个工厂生产的汽车数量
• 按car_model分组

输出应该是这样的：

| factory_id | car_model_id = 1 | car_model_id = 2 | car_model_id = ... |
+------------+------------------+------------------+--------------------+
| 1          |        145       |       3307       |         ...        |
+------------+------------------+------------------+--------------------+
| 2          |       32798      |        345       |         ...        |
+------------+------------------+------------------+--------------------+
| 3          |         0        |        10        |         ...        |
+------------+------------------+------------------+--------------------+
| ...        |        ...       |        ...       |         ...        |
+------------+------------------+------------------+--------------------+

我最接近的尝试是这个请求，但它仍然不是我想要的：
SELECT COUNT(car_id) FROM car GROUP BY car_factory_id AND car_model_id

我的RDBMS是MySQL 5.1。

感谢您的帮助：）

Answer 1

你通常不能通过行的值来参数化列的数量，用sql数据库查询你想要的东西的惯用方法是得到这样的东西：

factory_id | car_model_id | count
1          | 1            | 145
1          | 2            | 3307
...
4          | 1            | 3245
4          | 2            | 38

在这种情况下你会这样做：

SELECT factory_id, car_model_id, COUNT(*) FROM factory
INNER JOIN car ON factory_id = car_factory_id GROUP BY factory_id, car_model_id

Answer 2

试试这个，;）

SELECT  a.factory_name , c.car_model_name,
        COUNT(b.car_factory_id) totalCount
FROM    factory a
        LEFT JOIN car b
            ON a.factory_id = b.car_factory_id
        LEFT JOIN car_model c
            ON b.car_model_id = c.car_model_id
GROUP BY a.factory_name , c.car_model_name
ORDER BY factory_name ASC