I'd suggest to use the id and not the username, because if you start to use the username as a join column in multiple tables, you've to remember to update all of them.

The foreign key for the users table, becomes the primary key of addressdata table and the primary key must remain stable. It's a good practice to not change primary key fields. A primary key must exist when the record is created, and has to remain unchanged for the whole lifetime of the record.

If you'd like further insights The great primary-key debate is a great article.

What you are proposing to do can only be done with MySQL cleanly under three(3) conditions

• CONDITION #1 : Use the MyISAM storage engine
• CONDITION #2 : Make auto_increment column part of a compound primary key
• CONDITION #3 : Each auto_increment for a given type must exist in its own row
• See the auto_increment documentation for MyISAM

Here is your original table layout

CREATE TABLE `invoices` ( 
  `id` mediumint unsigned NOT NULL AUTO_INCREMENT PRIMARY KEY, 
  `invoicenumber` mediumint unsigned NOT NULL, 
  `branch` enum('A','B') NOT NULL, 
  `date` date NOT NULL, 
  `client` varchar(100) NOT NULL 
) COMMENT='' ENGINE='InnoDB'; 

Based on the three conditions I just mentioned, here is the new proposed table layout:

CREATE TABLE `invoices` ( 
  `invoicenumber` mediumint unsigned NOT NULL auto_increment, 
  `branch` enum('A','B') NOT NULL, 
  `date` date NOT NULL, 
  `client` varchar(100) NOT NULL,
  PRIMARY KEY (branch,invoicenumber)
) COMMENT='' ENGINE='MyISAM'; 

Here is an example via sample data and SQL:

drop database if exists user1162541;
create database user1162541;
use user1162541
CREATE TABLE `invoices` ( 
  `invoicenumber` mediumint unsigned NOT NULL auto_increment, 
  `branch` enum('A','B') NOT NULL, 
  `date` date NOT NULL, 
  `client` varchar(100) NOT NULL,
  PRIMARY KEY (branch,invoicenumber)
) COMMENT='' ENGINE='MyISAM'; 
INSERT INTO invoices (branch,date,client) VALUES
('A',DATE(NOW()),'John'),
('B',DATE(NOW()),'Jack'),
('A',DATE(NOW()),'Jeff'),
('B',DATE(NOW()),'Joel'),
('A',DATE(NOW()),'Jane'),
('B',DATE(NOW()),'Joan'),
('A',DATE(NOW()),'June');
SELECT * FROM invoices ORDER BY branch,invoicenumber;

Here it is executed:

mysql> drop database if exists user1162541;
Query OK, 1 row affected (0.01 sec)

mysql> create database user1162541;
Query OK, 1 row affected (0.02 sec)

mysql> use user1162541
Database changed
mysql> CREATE TABLE `invoices` (
    ->   `invoicenumber` mediumint unsigned NOT NULL auto_increment,
    ->   `branch` enum('A','B') NOT NULL,
    ->   `date` date NOT NULL,
    ->   `client` varchar(100) NOT NULL,
    ->   PRIMARY KEY (branch,invoicenumber)
    -> ) COMMENT='' ENGINE='MyISAM';
Query OK, 0 rows affected (0.00 sec)

mysql> INSERT INTO invoices (branch,date,client) VALUES
    -> ('A',DATE(NOW()),'John'),
    -> ('B',DATE(NOW()),'Jack'),
    -> ('A',DATE(NOW()),'Jeff'),
    -> ('B',DATE(NOW()),'Joel'),
    -> ('A',DATE(NOW()),'Jane'),
    -> ('B',DATE(NOW()),'Joan'),
    -> ('A',DATE(NOW()),'June');
Query OK, 7 rows affected (0.02 sec)
Records: 7  Duplicates: 0  Warnings: 0

mysql> SELECT * FROM invoices ORDER BY branch,invoicenumber;
+---------------+--------+------------+--------+
| invoicenumber | branch | date       | client |
+---------------+--------+------------+--------+
|             1 | A      | 2012-04-21 | John   |
|             2 | A      | 2012-04-21 | Jeff   |
|             3 | A      | 2012-04-21 | Jane   |
|             4 | A      | 2012-04-21 | June   |
|             1 | B      | 2012-04-21 | Jack   |
|             2 | B      | 2012-04-21 | Joel   |
|             3 | B      | 2012-04-21 | Joan   |
+---------------+--------+------------+--------+
7 rows in set (0.00 sec)

mysql>

Give it a Try !!!

CAVEAT : At present, only the MyISAM Storage Engine supports multiple auto_increment values grouped with other columns. This is not possible with InnoDB based on auto_increment columns being tied directly to the gen_clust_index (aka Clustered Index) !!!