Mysql GROUP_CONCAT 只返回一個值

Question

我嘗試通過GROUP_CONCAT返回用戶有多少朋友但我只得到Lance而不是Bob並且似乎如果我刪除WHERE條件。 它工作正常，但我想保留它。 因為我想獲取所有連接到member的行

CREATE TABLE users(
    id INT PRIMARY KEY AUTO_INCREMENT,
    name VARCHAR(255)
);

INSERT INTO users (name)
VALUES ("Gregor"),
    ("Liza"),
    ("Matt"),
    ("Tim"),
    ("Lance"),
    ("Bob");
    
CREATE TABLE committee(
    id INT PRIMARY KEY AUTO_INCREMENT,
    user_id INT,
    friend_id INT,
    member_id INT,
    FOREIGN KEY (`user_id`) REFERENCES `users` (`id`),
    FOREIGN KEY (`friend_id`) REFERENCES `users` (`id`),
    FOREIGN KEY (`member_id`) REFERENCES `users` (`id`)
);
INSERT INTO committee (user_id, friend_id, member_id)
VALUES (3, 5, 1),
(4, 5, 1),
(3, 6, 2),
(4, 6, 2);

這是查詢：

SELECT u.name,
       GROUP_CONCAT(f.name) AS friends
FROM committee c
INNER JOIN users u ON (u.id = c.user_id)
INNER JOIN users AS f ON (f.id = c.friend_id)
WHERE (c.member_id = 1)
GROUP BY u.id;

我現在得到的：

name    friends
Matt    Lance
Tim Lance

我的期望：

name    friends
Matt    Lance,Bob
Tim Lance,Bob

Answer 1

您需要再次加入committee才能找到馬特和蒂姆所在的所有其他委員會，這樣您就可以從這些委員會中找到他們的朋友。

SELECT u.name,
       GROUP_CONCAT(f.name) AS friends
FROM committee c
INNER JOIN users u ON (u.id = c.user_id)
INNER JOIN committee c2 ON c2.user_id = c.user_id
INNER JOIN users AS f ON (f.id = c2.friend_id)
WHERE (c.member_id = 1)
GROUP BY u.id;

演示