在 MySql 数据库中，我有一个相当常见的用户和组，其中有一个 pivot users_groups 以允许 N:M 关系。
表用户
id | name
--------+----------
1 | Joe
2 | Anna
3 | Max
表组
id | name
---------+----------
1 | Red
2 | Blue
3 | Green
表 users_groups
id | userid | groupid
---------+--------+---------
1 | 1 | 2
2 | 3 | 2
3 | 1 | 3
3 | 2 | 1
所以...红(1)组的成员是安娜(2)，绿(3)组的成员是乔(1)，蓝(2)组的成员是乔(1)和麦克斯(3).
当用户登录时，我有用户 ID(例如 1 代表 Joe)，我想查找我的登录用户也是其中成员的特定组中的所有其他用户。如何获取该组中的用户列表？
我需要使用表单提供的文本查找组名，用户 ID 将从授权/登录代码中获取。如果用户不属于群组，他们应该无法获得群组成员列表。
对于 Red 组，当以 Anna 身份登录时，我应该只能看到一个用户 (Anna)
User | Group | Users in Group must include the current user
------+------------
1 | Red | EMPTY
User | Group | Users in Group must include the current user
------+------------
2 | Red | Anna
User | Group | Users in Group must include the current user
------+------------
3 | Red | EMPTY
对于 Blue 组，如果我以 Joe 或 Max 身份登录，那么我应该会看到一个用户列表(Joe 和 Max)
User | Group | Users in Group must include the current user
------+------------
1 | Blue | Joe, Max
User | Group | Users in Group must include the current user
------+------------
2 | Blue | EMPTY
User | Group | Users in Group must include the current user
------+------------
3 | Blue | Joe, Max
对于 Green 组，当以 Joe 身份登录时，我应该只能看到一个用户 (Joe)
User | Group | Users in Group must include the current user
------+------------
1 | Green | Joe
User | Group | Users in Group must include the current user
------+------------
2 | Green | EMPTY
User | Group | Users in Group must include the current user
------+------------
3 | Green | EMPTY
===更新#1 ===
使用@Erico 的回答和下面的 fiddle 以及更新的表架构以包含已启用和电子邮件字段，我可以通过额外的 enabled 列检查来执行以下操作。但是，我想将所有用户作为结果集中的单独行返回，而不是返回包含所有数据的单个 Users 列。
http://sqlfiddle.com/#!9/80da98/2
SELECT '1' as User, name as Group,
(SELECT GROUP_CONCAT(email) FROM users u, users_groups ug
WHERE u.enabled = 1 AND u.id = ug.user_id AND ug.group_id = g.id AND ug.group_id
IN (SELECT group_id FROM users_groups WHERE user_id = 1)
) as Users
FROM groups g
WHERE g.name = 'Blue' AND g.enabled = 1
=== 更新 #2 ===
而不是在一行中返回结果:
User | Group | Users in Group must include the current user
------+------------
1 | Blue | Joe, Max
或使用电子邮件代替姓名
User | Group | Users in Group must include the current user
------+------------
1 | Blue | joe@mycompany.com, max@hiscompany.com
我想在一行中返回每个用户的完整信息，因此搜索 Group Blue(2) 中的用户 Joe(1) 将返回:
User | Name | Email
------+------------
1 | Joe | joe@mycompany.com
3 | Max | max@hiscompany.com
最佳答案
这是一个带有子查询的示例，显示的结果如您的问题所示。
Joe 正在显示蓝色组:
SELECT '1' as `User`, name as `Group`,
(SELECT GROUP_CONCAT(name) FROM users u, users_groups ug
WHERE u.id = ug.user_id AND ug.group_id = g.id
AND ug.group_id IN (SELECT group_id FROM users_groups WHERE user_id = 1)
) as `Users`
FROM groups g
WHERE g.name = 'Blue'
关于MySQL 仅返回指定组中的用户，我们在Stack Overflow上找到一个类似的问题： https://stackoverflow.com/questions/34407300/