Question

我有三张表公司，用户，地址。

公司有三个字段

comp_id（PK）
COMP_NAME
USER_ID（FK）
to_address_id（FK）
from_address_id（FK）

地址包含字段：
ADDRESS_ID（PK）
城市
状态

用户有字段 USER_ID（PK），USER_NAME

to_address_id和from_address_id都是外键，它引用相同的表地址并映射到address_id

现在重点是如何从表中检索from_address，我能够从此查询中检索to_address。

在这里，我可以从Shipment表中获取to_address_id和from_address_id，但是如何根据address_id获取同一查询中的两个地址：

 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 - 您需要加入两次，每次对于您感兴趣的每个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

您必须先查看在查询中使用 JOIN 。

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;

如何从外键表访问数据

3 个答案: