Bad MYSQL design and how to improve it - mysql

Question

I have the following problem tying to sort out a database which was when I started an unholy mix of Mysql, hand written back of envelope notes, Excel spreadsheet and completely missing records (don't ask) I have now reduced the problem down to the following.
I have 4 tables in greatly simplified form they are:-
customers
---------
id int not null primary key,
name varchar(50)
users
---------
id2 int not null primary key,
name varchar(50)
address
-------
id int,
id2 int,
country varchar(50)
product
-------
id int,
id2 int,
item varchar(50)
Sample data:
Examples select * from customers;
+----+------+
| id | name |
+----+------+
| 1 | Fred |
+----+------+
select * from users;
+----+---------+
|id2 | name |
+----+---------+
| 1 | Wilma |
| 2 | Pebbles |
+----+---------+
select * from address;
+----+-----+---------+
| id | id2 | country |
+----+-----+---------+
| 1 | 1 | Bedrock |
| 1 | 2 | Bedrock |
+----+-----+---------+
select * from product;
+----+-----+---------+
| id | id2 | item |
+----+-----+---------+
| 1 | 1 | Item1 |
| 1 | 2 | Item2 |
+----+-----+---------+
customers.id is a primary key and links to address.id and product.id
users.id2 is a primary key and links to address.id2 and product.id2
This arrangement fails where more than one user shares an address with a customer the only way to work around this at present is to duplicate the record in address and change the id2 number. At present this only effects one case in the database.
Where users don't share an address with a customer I am unable to workout a select statement that will find Customers name, Customers address, Users name and Users address.
This situation applies to approximately 30% of the records.
Any suggestions on how to sort this chaos would be most welcome.
Richard

Answer 1

To summarize what I think you're describing: An address can be shared by multiple users, but a user/customer can only have one address.
It sounds like you have your many-to-one relationship backwards.
Meaning, you have a foreign key in the address table pointing to customer/user, when what you really want is a foreign key in the customer/user table pointing to address. But that's kind of ugly... so a couple of ways you might clean it up:
Is a customer also a user? If so, have a single foreign key in Address that points to User, and a foreign key from User to Customer would be fairly clean.
If not, you might consider a table that bridges User/Customer and address. Something like this:
User_Cust_ID, Type, Address_ID
1 Customer 5
2 Customer 1
1 User 1
That will allow you to resolve a user and customer that share the same address.
HTH...