MySQL 查询返回不同的结果

Question

我对以下查询有一个奇怪的问题：

SELECT
    doubles_team_members.doubles_team_id,
    GROUP_CONCAT(CONCAT(members.first_name,' ',members.last_name) SEPARATOR ' & ') team,
    GROUP_CONCAT(DISTINCT CONCAT(members.city,', ',members.state) SEPARATOR ' & ') location,
    doubles_teams.deleted_by_id
FROM
    doubles_team_members
JOIN
    doubles_teams USING(doubles_team_id)
JOIN
    members USING(member_id)
JOIN
    memberships ON members.member_id=memberships.member_id
WHERE
    (memberships.expiration_date>=CURDATE() OR members.lifetime=1) AND 
    (doubles_teams.deleted_by_id IS NULL OR doubles_teams.deleted_by_id=0) AND 
    (members.deleted_by_id IS NULL OR members.deleted_by_id=0)
GROUP BY
    doubles_team_members.doubles_team_id
HAVING
    team='Van Williams & Derek Williams';

有时我会得到零、一、二或三个结果。我应该总是得到三个结果。这可能是什么原因造成的？

Answer 1

正如Solarflare 所指出的，GROUP_CONCAT() 可以以随机顺序生成您的team 字符串，例如'Van Williams & Derek Williams' 或 'Derek Williams & Van Williams'。

要解决这个问题，您可以use an explicit ORDER BY within the GROUP_CONCAT()，例如：

GROUP_CONCAT(CONCAT(members.first_name,' ',members.last_name) ORDER BY first_name DESC SEPARATOR ' & ') team

有关示例，请参阅 documentation 和 this db<>fiddle。