Question

It’s easy to create a database table for storing sequence numbers ; but this design is suited for the event when the sequence is shared for all users. What I want is to create sequence for each group of users : this group can grow at any time because it’s a database table , that is the administrator can create a group at any time and users are assigned to a specific group. So how to implement the sequence generation according to a group ?

Answer 1

if you are using myisam

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

Below extracted from above links.

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 |
+--------+----+---------+

For your case:

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;

Returns:

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