Multiple values for unique id? I have two tables: customer and boxes in one-to-many relationship.
The structure of the tables is in addition :

Customer table :   id#| name | date_purchased | address |
                    1    John     2012-9-10       N/A

Boxes table:   id#| type | quantity |
                1    big      4 


The id of the boxes table is foreign key to the customer table.

What if John has purchased more than 4 big boxes ? For example: he has ordered 2small boxes as well. How to add this to the database design schema?
I would create three tables instead:

Customer table:


Customer_id, name, address, etc. 
PK: customer_id


Boxes table:


Box_id, size, description, etc. 
PK: box_id


Orders table:


Customer_id, Box_id, Quantity, Timestamp. 
PK: all four fields.
Foreign keys box_id and customer_id to the respective tables.


You will have one entry for each customer, each box type, and as many boxes of different sizes ordered as you wish.