如何访问外键表中的数据

Question

我有三个表 Company, User , Address 。

company 有三个字段

• comp_id(PK)
• comp_name
• user_id(FK)
• to_address_id(FK)

• from_address_id(FK)

  地址有字段：

• address_id(PK)
• 城市

• 状态

  用户有字段 user_id(PK),用户名

  to_address_id 和 from_address_id 都是外键，引用同一个表地址并映射到 address_id

  现在的重点是如何从表中检索 from_address，我可以从该查询中检索 to_address。

  这里我可以从 Shipment 表中获取 to_address_id 和 from_address_id，但是如何根据 address_id 在同一个查询中获取两个地址：

 SELECT s.comp_name,u.user_name,a.city
            FROM
        company s
    JOIN
        User u
    JOIN
        Address a
    ON
        s.user_id = u.user_id 
        AND s.to_address_id = a.address_id
    WHERE
        s.user_id = 1001;

Answer 1

解决方案 - 您需要 JOIN 两次，每次针对您感兴趣的每个 FK：

SELECT s.comp_name, u.user_name, a_from.city AS city_from, a_to.city AS city_to
FROM company s
LEFT JOIN User u ON s.user_id = u.user_id 
LEFT JOIN Address a_from ON s.from_address_id = a_from.address_id
LEFT JOIN Address a_to ON s.to_address_id = a_to.address_id
WHERE s.user_id = 1001;

Answer 2

尝试以下方法：

 SELECT s.comp_name
        ,u.user_name
        ,a1.city
        ,a2.city 
 FROM company s 
 JOIN User u ON s.user_id = u.user_id  
 JOIN Address a1 ON s.to_address_id = a1.address_id 
 JOIN Address a2 ON s.from_address_id = a2.address_id
 WHERE s.user_id = 1001;

Answer 3

你要先看看JOIN在查询中的使用。

试试这个：

SELECT s.comp_name,u.user_name,a.city
FROM company s
JOIN User u ON s.user_id = u.user_id 
JOIN Address a ON s.to_address_id = a.address_id
WHERE s.user_id = 1001;