假设,我有两个这样的表:
Table group Table user
+----+-----------+ +----+----------+------+----------+
| id | groupname | | id | username | rank | group_id |
+----+-----------+ +----+----------+------+----------+
| 1 | Friends | | 1 | Frank | 1 | 1 |
| 2 | Family | | 2 | Mike | 3 | 1 |
+----+-----------+ | 3 | Steve | 2 | 1 |
| 4 | Tom | 1 | 2 |
+----+----------+------+----------+
我想选择所有组并获取每个组中排名最高(最高数字)的用户。所以基本上我想得到这个结果:
+-----------------+----------+---------+---------------+
| group.groupname | group.id | user.id | user.username |
+-----------------+----------+---------+---------------+
| Friends | 1 | 2 | Mike |
| Family | 2 | 4 | Tom |
+-----------------+----------+---------+---------------+
选择怎么样? 也许很简单,但我现在还没有明白......
最佳答案
编辑2:
我之前的答案是错误的,max()
调用破坏了结果。这是一个正确的解决方案:
SELECT g.groupname, g.id AS group_id, u.id AS user_id, u.username
FROM `user` u
LEFT JOIN `group` g ON (u.group_id=g.id)
WHERE u.rank=(
SELECT MAX(rank)
FROM `user` u2
WHERE u.group_id=u2.group_id
)
WHERE
子句中的检查也应该更容易理解。
mysql> SELECT g.groupname, g.id AS group_id, u.id AS user_id, u.username
-> FROM `user` u
-> LEFT JOIN `group` g ON (u.group_id=g.id)
-> WHERE u.rank=(
-> SELECT MAX(rank)
-> FROM `user` u2
-> WHERE u.group_id=u2.group_id
-> );
+-----------+----------+---------+----------+
| groupname | group_id | user_id | username |
+-----------+----------+---------+----------+
| Friends | 1 | 2 | Mike |
| Family | 2 | 4 | Tom |
+-----------+----------+---------+----------+
2 rows in set (0.00 sec)
关于mysql - mysql 查询必须如何才能达到显示的结果?,我们在Stack Overflow上找到一个类似的问题: https://stackoverflow.com/questions/2117903/