Question

我实际上并不需要以下查询，但我醒过这个理论问题＆＃34;我很难搞清楚。假设我有三个表：用户表，组表和users_groups表，它是多对多的。因此，如果一个用户属于第1组和第2组，则每个用户将有两个不同的行。

现在，假设有很多组，我该如何具体选择属于第2组和第3组的用户？例如？

我沿着这些方向尝试了一些东西，但它显示为空：

SELECT * FROM `users_groups` GROUP BY user_id HAVING group_id = 2 AND group_id = 3

我想这假设两个组都在同一行，这显然不会起作用。我该怎么做？

编辑：两种变体如何：用户必须只在这两个组中，并且用户必须至少在这两个组中？

Answer 1

SELECT *
FROM users_groups
GROUP BY user_id
WHERE group_id IN (2,3)
HAVING COUNT(1) = 2

这当然假设{user_id，group_id}是唯一的（并且没有其他列可以向计数添加其他行）。否则你可以明确地确保这一点：

SELECT *
FROM users_groups
GROUP BY user_id
WHERE group_id IN (2,3)
HAVING COUNT(DISTINCT group_id) = 2

这两组中只有稍微复杂一些。你可以这样做：

SELECT *
FROM users_groups g1
GROUP BY user_id
WHERE group_id IN (2,3)
AND NOT EXISTS
(
    SELECT 1
    FROM users_groups AS g2
    WHERE g2.user_id = g1.user_id
    AND group_id NOT IN (2,3)
)
HAVING COUNT(1) = 2

或者，

SELECT *
FROM users_groups g1
GROUP BY user_id
HAVING COUNT(1) = 2
AND SUM(CASE WHEN group_id IN (2,3) THEN 1 ELSE 0 END) = 2

在第2组和第3组中，总共超过2组：

SELECT *
FROM users_groups g1
GROUP BY user_id
HAVING SUM(CASE WHEN group_id IN (2,3) THEN 1 ELSE 0 END) = 2
AND COUNT(1) > 2

Answer 2

问题称为Relational Division。

SELECT  a.ID, a.Name
FROM    users a
        INNER JOIN users_groups b
            ON a.ID = b.UserID
        INNER JOIN groups c
            ON b.group_ID = c.ID
WHERE   c.Name IN ('grp2', 'grp3')
GROUP   BY a.ID, a.Name
HAVING  COUNT(DISTINCT c.Name) = 2

如果DISTINCT上的唯一约束不是针对每个用户强制执行，则在以下查询中使用

Name，否则HAVING COUNT(*) = 2就足够了。

SQL of Relational Division

Answer 3

  SELECT *, COUNT(*) FROM `users_groups` 
  WHERE group_id IN (2,3) 
  GROUP BY user_id HAVING COUNT(*) > 1

Answer 4

用户必须仅属于第2组和第3组：

SELECT *, group_concat(group_id ASC) gui
FROM users_groups
GROUP BY user_id
HAVING gui="2,3"

Answer 5

SELECT user_id FROM（select * from users_groups where group_id = 2）grp 1，（select * from users_groups where group_id = 3）grp2 其中grp1.user_id = grp2.user_id

如何从两组中的多对多表中选择用户？

5 个答案: