Question

不确定这是否可以在DB端进行，但到目前为止我只能得到这个结果：

查询：

SELECT City.city_name "City", PC.subcategory_id "Subcategory", PC.count "count" FROM products_counter PC , Cities City WHERE PC.city_id = City.city_id

+-----------+----------------+-------+
| city_name | subcategory_id | count |
+----------------------------+-------+
| City1     | fruits         | 4     |
| City2     | vegetables     | 4     |
| City1     | meat           | 1     |
+-----------+----------------+-------+

以下是我的两张表：

表 products_counter：

+-------+---------+----------------+-------+
| ID    | city_id | subcategory_id | count |
+-------+---------+----------------+-------+
|     1 |       1 | fruits         |     4 |
|     2 |       2 | vegetables     |     4 |
|     3 |       2 | meat           |     1 |
+-------+---------+----------------+-------+

表城市：

+---------+------------+
| city_id | city_name  |
+---------+------------+
|       1 | City1      |
|       2 | City2      |
|       3 | City3      |
+---------+------------+

这是预期的结果：

+-----------+----------------+-------+
| city_name | subcategory_id | count |
+-----------+----------------+-------+
| City1     | fruits         | 4     |
| City1     | vegetables     | 0     | 
| City1     | meat           | 0     |
| City2     | fruits         | 0     |
| City2     | vegetables     | 4     |
| City1     | meat           | 1     |
| City3     | fruits         | 0     |
| City3     | vegetables     | 0     |
| City3     | meat           | 0     |
+-----------+----------------+-------+

但我不确定如何列出Cities表中的所有城市，如果city_id和subcategory_id相等，则只分配count列。

Answer 1

试试这个。

select  c.city_name  ,
        ps.subcategory_id ,
        (   select  `count` 
            from    products_counter 
            where   city_id = c.city_id 
                and subcategory_id = ps.subcategory_id) as 'Count'
from       cities           c
cross join products_counter pc

Answer 2

您可以使用交叉联接。

SELECT c.city_name, pc.subcategory_id,
IFNULL((select `count` from products_counter where city_id = c.city_id 
                and subcategory_id = pc.subcategory_id),0) as 'Count'
FROM cities c CROSS JOIN products_counter pc

这里的工作示例 - http://sqlfiddle.com/#!9/34c38/16

Answer 3

select city_name, subcategory_id,
case when cities.city_id = pc.city_id then `count` else 0 end as counter
from cities, products_counter pc

mysql组合来自两个表的数据

3 个答案: