Mysql GROUP_CONCAT只返回一个值。

Question

我试图返回一个用户通过GROUP_CONCAT有多少朋友，但我只得到了Lance，也没有Bob，而且似乎如果我删除了WHERE条件。它很好，但我想留着它。因为我想获取连接到member的所有行

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);

以下是查询：

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;

我现在得到的是：

name    friends
Matt    Lance
Tim Lance

我所期望的：

name    friends
Matt    Lance,Bob
Tim Lance,Bob

Answer 1

你需要再和committee一起找到马特和提姆所在的其他委员会，这样你就可以从这些委员会找到他们的朋友了。

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