Question

我有两张桌子：

g_relationships (id,request_id,receiver_id,status,approved)
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 获取用户名并将其添加到临时列中。

例如：假设我有 userid 4。然后我想选择 g_relationships，并与 g_users 保持连接，并添加一列检查我的 userid (4) 是否在 request_id 或receiver_id 列中，并获取 userid这不是我的，并检查它是哪个用户名并用它填充新列。

例如：正确的结果应该是这样的：（如果我收到好友请求）

<头>

id	request_id	receiver_id	状态	已批准	friend(new_column)
1	3	4	朋友	no	Neo

但是如果我是发送好友请求的人，它会是这样的

<头>

id	request_id	receiver_id	状态	已批准	friend(new_column)
1	4	3	朋友	no	Neo

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，并检查 g_users 表并将用户名从那里放入朋友列。

这是我到目前为止想出的，但这只会检查 request_id 不是 4，并且也不会使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。两者都使用额外的 JOIN 到 g_users 表。第一个是单个查询，可让您获得所需的输出，而第二个（在我看来）更具可读性，但使用 UNION 并且从技术上讲是两个查询。

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");

超级连接：

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;

联合：

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 - 待朋友批准/用户和朋友之间的关系

2 个答案: