mysql - MySQL：根据多行数据组合多个相关表

Question

[更新]

我有三个由一个表定义的表，这三个表之间的关系由另一个表定义，如下所示：

1. 表 #1：wp_pods_cities [城市 PODS]

   item_id  city
   -------- ----------
   1        Albany
   2        Perth
   3        Albany
   4        Hollywood
   5        Albany
   6        San Diego
   7        Denpasar
   

2. 表 #2：wp_pods_regions [区域 PODS]

   item_id  region
   -------- ------------------
   1        Western Australia
   2        California
   3        Texas
   4        Bali
   

3. 表 #3：wp_pods_countries [国家 PODS]

   item_id  country
   -------- --------------
   1        Australia
   2        United States
   3        Indonesia
   

4. 表#4：wp_posts [帖子类型]

   pod_id  pod_name   post_type
   ------- ---------- ----------
   pod_1   countries  pods
   pod_2   regions    pods
   pod_3   cities     pods
   

5. 表#5：wp_podsrel [PODS 关系]

   rel_id  pod_id  item_id  related_pod_id  related_item_id
   ------- ------- -------- --------------- ----------------
   1       pod_2   1        pod_1           1 
   2       pod_2   2        pod_1           2 
   3       pod_2   3        pod_1           2 
   4       pod_2   4        pod_1           3 
   5       pod_3   1        pod_2           1 
   6       pod_3   2        pod_2           1 
   7       pod_3   3        pod_2           2 
   8       pod_3   4        pod_2           2 
   9       pod_3   5        pod_2           3 
   10      pod_3   6        pod_2           3
   11      pod_3   7        pod_2           4
   

注意：上面的表格是由PODS CMS 插件在Wordpress中创建的，但经过简化以使其在此处可读。实际的表也有数千条数据。

我想做的事

我想列出已定义的相同城市（在本例中为Albany）以及它们所属的地区和国家。

• 预期结果：

  city    region             country
  ------- ------------------ --------------
  Albany  Western Australia  Australia
  Albany  California         United States
  Albany  Texas              United States
  

我试过的

SELECT ct.city, rg.region, ctr.country
FROM wp_pods_cities AS ct
INNER JOIN wp_podsrel AS rel ON ct.item_id = rel.item_id
AND rel.pod_id IN ('pod_3', 'pod_2')
INNER JOIN wp_pods_regions AS rg ON rel.related_item_id = rg.item_id
INNER JOIN wp_pods_countries AS ctr ON rel.related_item_id = ctr.item_id
WHERE ct.city = 'Albany'
ORDER BY ct.city

这将为我提供城市及其相关地区，但国家不相关。如何实现上述示例结果？

先谢谢了。

Answer 1

我设法通过两次使用 podsrel 表来获得正确的输出，一次用于城市，一次用于区域。

SELECT ct.city, rg.region, co.country

FROM wp_pods_cities AS ct, wp_podsrel AS pr

INNER JOIN wp_pods_regions AS rg
    ON rg.item_id = pr.related_item_id

INNER JOIN wp_podsrel AS pr2
    ON pr2.item_id = rg.item_id

INNER JOIN wp_pods_countries AS co
    ON co.item_id = pr2.related_item_id

WHERE ct.city = 'Albany'
    AND pr.item_id = ct.item_id

ORDER BY ct.city

Answer 2

感谢@sjdaws的输入，我设法为我更新的问题修改了他的解决方案。

SELECT ct.city, rg.region, co.country
    FROM wp_pods_cities AS ct, wp_podsrel AS pr
        INNER JOIN wp_pods_regions AS rg
            ON rg.item_id = pr.related_item_id
        INNER JOIN wp_podsrel AS pr2
            ON pr2.item_id = rg.item_id
        INNER JOIN wp_pods_countries AS co
            ON co.item_id = pr2.related_item_id
    WHERE ct.city = 'Albany'
        AND pr.item_id = ct.item_id
        AND pr.pod_id = 'pod_3'
        AND pr2.item_id = rg.item_id
        AND pr2.pod_id = 'pod_2'
    ORDER BY co.country, rg.region

在这里查看结果：http ://www.sqlfiddle.com/#!2/d40c0/10