Question

表用户：

id----email 
1-----s@s.com 
2-----p@p.com

表user_buddies

id-----user_id----buddy_id----status enum('accepted','pending') 
1------1----------2-----------pending

假设我正在查询用户2，那么我想获得用户2的电子邮件。如果我正在查询用户1，那么我想收到用户1的电子邮件。

以下查询始终返回空行。

SELECT
    users.email,
    ub1.*, ub2.*
FROM
    users
JOIN user_buddies ub1 ON users.id = ub1.user_id
JOIN user_buddies ub2 ON users.id = ub2.buddy_id
WHERE
    users.id = 1;

任何人都可以帮助我吗？谢谢。

预期结果：

email-----user_id-----buddy_id-----status
s@s.com --1-----------2------------pending (If queryed on user 2)
p@p.com---1-----------2------------pending (If queried on user 1)

上下文

当user1向user2 发送请求时，只有一行添加到user_buddies表status=pending。因此，user1和user2伙伴列表都是使用相同的行创建的。因此，如果查询user2所需的电子邮件是user2's。

Answer 1

您可以查看以下查询：

SELECT
    users.email,
    ub1.user_id,
    ub1.buddy_id,
    ub1.`status`
FROM
    users
JOIN user_buddies ub1 ON users.id = ub1.user_id OR users.id = ub1.buddy_id
WHERE   users.id = 2;

SQL FIDDLE DEMO

<强>结果：

email      user_id     buddy_id     status
p@p.com       1           2         pending (if queried on users.id = 2)
s@s.com       1           2         pending (if queried on users.id = 1)

修改

SELECT ( SELECT u.email FROM users u WHERE u.id = IF ( users.id = ub1.user_id, ub1.buddy_id, ub1.user_id ) ) AS buddyEmail, ub1.user_id, ub1.buddy_id, ub1.`status` FROM users INNER JOIN user_buddies ub1 ON users.id = ub1.user_id OR users.id = ub1.buddy_id WHERE users.id = 2;

Check this fiddle of this new query

Answer 2

SELECT 'Userid' querysource, u.id, u.email, ub.*, u1.EMAIL AS Buddyemail
FROM @users u 
JOIN @user_buddies ub ON u.id = ub.user_id
JOIN @users u1 ON u1.id = ub.BUDDY_ID
WHERE u.id = 1

UNION

SELECT 'Buddyidid', u.id, u.email, ub.*, u1.email AS Buddyemail
FROM @users u 
JOIN @user_buddies ub ON u.id = ub.buddy_id 
JOIN @users u1 ON u1.id = ub.user_ID
WHERE u.id = 1

如何编写mysql查询来实现以下类型的连接（基于列的值加入）

2 个答案: