mysql - 如何实现每组用户的序号数据库？

Question

创建用于存储序列 数字的数据库表很容易；但这种设计适用于 sequence 为所有用户共享的事件。我想要的是为每个 group 用户创建 sequence :这个 group 可以随时增长，因为它是一个数据库表，就是管理员可以随时创建一个组，并将用户分配到一个特定的组。那么如何实现按组生成sequence呢？

Answer 1

如果你使用的是myisam

http://dev.mysql.com/doc/refman/5.0/en/example-auto-increment.html

下面是从上面的链接中提取的。

For MyISAM and BDB tables you can specify AUTO_INCREMENT on a secondary column in a multiple-column index. In this case, the generated value for the AUTO_INCREMENT column is calculated as MAX(auto_increment_column) + 1 WHERE prefix=given-prefix. This is useful when you want to put data into ordered groups.

CREATE TABLE animals (
    grp ENUM('fish','mammal','bird') NOT NULL,
    id MEDIUMINT NOT NULL AUTO_INCREMENT,
    name CHAR(30) NOT NULL,
    PRIMARY KEY (grp,id)
) ENGINE=MyISAM;

INSERT INTO animals (grp,name) VALUES
    ('mammal','dog'),('mammal','cat'),
    ('bird','penguin'),('fish','lax'),('mammal','whale'),
    ('bird','ostrich');

SELECT * FROM animals ORDER BY grp,id;
Which returns:

+--------+----+---------+
| grp    | id | name    |
+--------+----+---------+
| fish   |  1 | lax     |
| mammal |  1 | dog     |
| mammal |  2 | cat     |
| mammal |  3 | whale   |
| bird   |  1 | penguin |
| bird   |  2 | ostrich |
+--------+----+---------+

针对您的情况:

CREATE TABLE mytable (
   user_id MEDIUMINT NOT NULL AUTO_INCREMENT,
    group_id MEDIUMINT NOT NULL,   
     user_name CHAR(30) NOT NULL,
    PRIMARY KEY (group_id,user_id)
     ) ENGINE=MyISAM;


INSERT INTO mytable (group_id, user_name) VALUES
     (1,'alex'),(1,'jenny'),(2,'baz'),(1,'tim'),(2,'danny'),(3,'joe');


SELECT * FROM mytable ORDER BY group_id,user_id;

返回:

user_id group_id    user_name
1            1          alex
2            1          jenny
3            1          tim
1            2          baz
2            2          danny
1            3          joe