Referencing table with generated primary keys - mysql

Question

I am trying to increase the constraint of my MySQL database schema adding foreign key constraint to each table.
Table 1: USERS
+---------+----------+-------------
| id | username | Other fields
+---------+----------+-------------
| 1 | John |
| 2 | Mark |
+---------+----------+-------------
id INT(11) UNSIGNED NOT NULL AUTO_INCREMENT
username` VARCHAR(50) NOT NULL
PRIMARY KEY (id)
Table 2: DISKS (This has a one to many relationship with USERS)
+---------+----------+-----------+-------------
| id | id_user | disk_name | Other fields
+---------+----------+-----------+-------------
| 1 | 1 | disk A |
| 2 | 2 | disk B |
+---------+----------+-----------+-------------
id INT(11) UNSIGNED NOT NULL AUTO_INCREMENT
id_user INT(11) NOT NULL,
PRIMARY KEY (id,id_user)
INDEX fk_disks_idx (id ASC)
CONSTRAINT fk_disks
FOREIGN KEY (id)
REFERENCES database.USERS (id)
ON DELETE NO ACTION
ON UPDATE NO ACTION)
Table 3: FILES (This has a one to many relationship with DISKS)
+---------+----------+----------+-----------+-------------
| id | id_disk | id_user | file_name | Other fields
+---------+----------+----------+-----------+-------------
| 1 | 1 | 1 | |
| 2 | 2 | 2 | |
+---------+----------+----------+-----------+-------------
id INT(11) UNSIGNED NOT NULL AUTO_INCREMENT
id_user INT(11) NOT NULL
id_disk INT(11) NOT NULL
PRIMARY KEY (id,id_disk,id_user )
INDEX fk_files_idx (id ASC, id_user ASC)
CONSTRAINT fk_files
FOREIGN KEY (id_disk, id_user, id_user)
REFERENCES database.DISKS (id)
ON DELETE NO ACTION
ON UPDATE NO ACTION)
Table 2: FILES_ON_NAS (This has a one to one relationship with FILES)
+-------+----------+----------+----------+-----------+-------------
| id | id_files | id_user | id_disk | file_name | Other fields
+-------+----------+----------+----------+-----------+-------------
| 1 | 1 | 1 | 1 | |
| 2 | 1 | 2 | 2 | |
+-------+----------+----------+----------+-----------+-------------
id INT(11) UNSIGNED NOT NULL AUTO_INCREMENT
id_files INT(11) NOT NULL,
id_user INT(11) NOT NULL,
id_disk INT(11) NOT NULL,
PRIMARY KEY (id,id_files,id_user,id_disk )
INDEX fk_files_on_nas_idx (id ASC)
CONSTRAINT fk_files_on_nas
FOREIGN KEY (id_files,id_user,id_disk)
REFERENCES database.FILES (id,id_user, id_disk)
ON DELETE NO ACTION
ON UPDATE NO ACTION)
Question:
As you can see the more I reference table in cascade the more primary keys I get. How can I design the database to avoid the replication of primary keys and therefore data duplication as well? Should I delete the auto incremented key for each table? Is it a good practice?
Thanks

Answer 1

The ID of the disk is sufficient to uniquely identify a disk. So there's no reason to include the ID of the user into the disk's primary key. It would even be an extremely bad idea, because that means that if a disk's user changes, you would need to modify the primary key.
Same for a file. A file ID uniquely identifies a file. So there's no reason to add the disk ID into the primary key of a file.

Answer 2

I would strongly advise against deleting the auto-incremented keys.
However you don't need to make a new primary key everytime :
If you want that multiple users share one disk, just put a foreign key of id_disk in USERS
If you want that one user can have multiple disks, then put a foreign key of id_user in DISKS instead.
Only use primary keys like that when you face a Many-to-Many relationship. In this case, you need to create a new table to join both tables; with the primary keys of both tables as foreign keys making a coupled primary key like you did.

Answer 3

You might want to read a little about Database Normalization. In your case, i would make the surrogate key id the only primary key in the tables. Something like:
create table users (
id integer not null auto_increment,
username varchar(50),
...,
primary key (id)
);
create table disks (
id integer not null auto_increment,
user_id integer,
diskname varchar(50),
....,
primary key (id),
foreign key (user_id) references users (id)
);
For files you are going to have to answer the question: does file ownership depend on the file directly, or transitively on the disk ownership, or are the ownerships independent? A file owned by John on a disk owned by Jack? Seems ok to me, but your domain might have different rules. In that case, drop the user_id from the files table (otherwise your database won't be in Third normal form).
create table files (
id integer not null auto_increment,
disk_id integer,
user_id integer, -- you have to decide whether this is necessary
filename varchar(50),
....,
primary key (id),
foreign key (disk_id) references disks (id),
foreign key (user_id) references users (id)
);