MySQL 查询 - 待批准好友/用户与好友之间的关系

Question

我有两个table：

g_relationships (id,request_id,receiver_id,状态,已批准)
g_users（其中包含：id、用户名...）

我很难进行添加临时列的查询，检查 userid: 4 是否存在于 request_id 或 receiver_id 列中，并获取另一个用户 id 并从 g_users 并将其添加到临时列。

例如：假设我的用户 ID 为 4。然后我想 select g_relationships，并与 g_users 左连接，并添加一个列来检查我的用户 ID ( 4) 在 request_id 或 receiver_id 列中，并获取不是我的用户 ID，并检查它是哪个用户名并填充新列。

例如：正确的结果应该是这样的：（如果我收到好友请求）

id	request_id	receiver_id	状态	批准	朋友(new_column)
1	3	4	朋友	没有	新

但如果我是发送好友请求的人，它会看起来像这样

id	request_id	receiver_id	状态	批准	朋友(new_column)
1	4	3	朋友	没有	新

换句话说，查询应该始终检查我没有出现的列，并获取该用户 ID，然后检查 g_users table 并将那里的用户名放入 friend列。

这是我到目前为止的想法，但这只检查 request_id 不是 4，也不会用用户名填充临时列“friend”:/

    SELECT *, ("friend" = CASE WHEN request_id != 4 THEN  END) as friend_id  
    FROM `g_relationships`
    LEFT JOIN g_users
    ON g_relationship.request_id = g_users.id
    WHERE request_id = 4 OR receiver_id = 4
    AND status = "friend"
    AND approved = "no"

有什么建议吗？

Answer 1

你试过这个吗？它将加入两个 ID 中的任何一个。

...
LEFT JOIN g_users ON ( g_relationship.request_id = g_users.id OR
                       g_relationship.receiver_id = g_users.id )  
WHERE g_users.id = 4

Answer 2

我写了这两种方式。两者都对 g_users table 使用额外的 JOIN。第一个是一个单一的查询，可以让你得到你正在寻找的输出，而第二个（在我看来）更具可读性但使用 UNION 并且在技术上是两个查询。

它们给出相同的输出。

沙盒设置：

CREATE TABLE g_relationships (
    id INT PRIMARY KEY,
    request_id INT,
    receiver_id INT,
    status VARCHAR(10),
    approved VARCHAR(10)
);

CREATE TABLE g_users (
    id INT PRIMARY KEY,
    username VARCHAR(20)
);

INSERT INTO g_users (id, username) VALUES
(1, "Jane"),
(2, "Bob"),
(3, "Sally"),
(4, "Frank");

INSERT INTO g_relationships (id, request_id, receiver_id, status, approved) VALUES
(1, 1, 3, "Friend", "No"),
(2, 1, 4, "Enemy", "No"),
(3, 4, 3, "Friend", "Yes"),
(4, 4, 2, "Friend", "Yes");

超级加入：

SELECT u.id, rel.request_id, rel.receiver_id, rel.status, rel.approved, u2.username AS FriendName
FROM g_relationships AS rel
JOIN g_users AS u
  ON (rel.request_id = u.id OR rel.receiver_id = u.id)
JOIN g_users AS u2
  ON (rel.request_id = u2.id OR rel.receiver_id = u2.id)
WHERE u.id != u2.id
  AND u.id = 4;

联盟：

SELECT u.id, rel.request_id, rel.receiver_id, rel.status, rel.approved, u2.username AS FriendName
FROM g_relationships AS rel
JOIN g_users AS u
  ON rel.request_id = u.id
JOIN g_users AS u2
  ON rel.receiver_id = u2.id
WHERE u.id = 4

UNION

SELECT u.id, rel.request_id, rel.receiver_id, rel.status, rel.approved, u2.username AS FriendName
FROM g_relationships AS rel
JOIN g_users AS u
  ON rel.receiver_id = u.id
JOIN g_users AS u2
  ON rel.request_id = u2.id
WHERE u.id = 4;

MySQL 查询 - 待批准好友/用户与好友之间的关系

MySQL Query - Pending Friends Approval / Relationship between user & friends

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