Mysql DISTINCT仍然回傳重複值

Question

如何避免在 friends 上重複，我仍然得到兩個 bob，而不是只有一個 bob

我的桌子設定：

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

INSERT INTO users (id, name)
VALUES (1, "Gregor"),
    (2, "Liza"),
    (3, "Matt"),
    (4, "Tim"),
    (5, "Lance"),
    (6, "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),
(3, 6, 2),
(4, 6, 2);

我使用的查詢：

SELECT DISTINCT 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;

當前結果：

name    friends
Matt    Lance,Bob,Bob
Tim Lance,Bob

我的期望：

name    friends
Matt    Lance,Bob
Tim Lance,Bob

Answer 1

您的 u.name 與 f.name 不同

試試這個

SELECT u.name,
       GROUP_CONCAT(distinct 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.name;

Answer 2

您只需要在 GROUP_CONCAT() 中使用 DISTINCT：

SELECT u.name,
       GROUP_CONCAT(DISTINCT f.name) AS friends
................................................

請注意，SELECT DISTINCT ... 在您的查詢中沒有意義，因為您使用的是 GROUP BY ，它為每個使用者傳回不同的行。

查看演示。