MySQL Categories and Subcategories Table Structure - mysql

Question

I am having a little bit of a problem with setting up a mysql table that will hold a list of categories and subcategories. I am not to sure how to setup the table. Does it need to be 2 separate tables? 1 for the main categories and 1 for the subcategories or can it be all in 1 table? Would something like this work?
Create Table categories (
category_id INT UNSIGNED NOT NULL AUTO_INCREMENT,
sub_id INT UNSIGNED NOT NULL,
name VARCHAR(100) NOT NULL,
PRIMARY KEY (category_id)
)
CREATE TABLE items (
item_id INT UNSIGNED NOT NULL AUTO_INCREMENT,
name VARCHAR(255) NOT NULL,
description VARCHAR(100) NOT NULL,
PRIMARY KEY (item_id),
FOREIGN KEY (category_id) REFERENCES categories (category_id),
FOREIGN KEY (sub_id) REFERENCES categories (sub_id)
)
Will this work or is this completely wrong? Thanks in advance for the help!

Answer 1

If you are 100% sure that you'll only have two levels of categories (main and sub), you can do a few different things. None of them are your proposed solution:
CREATE TABLE categories (
id int not null primary key,
main varchar(64)
sub varchar(64)
);
CREATE TABLE objects (
id int not null primary key,
category_id int,
name varchar(64),
FOREIGN KEY (category_id) REFERENCES categories (id)
);
Want all vehicles?
SELECT *
FROM objects AS o
INNER JOIN categories AS c ON o.category_id = c.id
WHERE c.main = 'vehicles';
Want all roflcopters?
SELECT *
FROM objects AS o
INNER JOIN categories AS c ON o.category_id = c.id
WHERE c.main = 'vehicles' and c.sub='Roflcopters';
If you want something in the "vehicle" category, but not in any of the subcategories of verhicles, just have a category record where main='vehicles' with sub NULL.
Of course, this is not particularly flexible. You're stuck with just two levels of categorization, and there's not a lot of business logic embedded in your category model. But it might be sufficient for your needs.
Two other good, proven, models are the adjacency list model, and the nested set model, both of which are described, with lots of nice example mysql code, over here

Answer 2

It depends.
Are categories and subcategories really two different things? This means categories have no parent, while subcategories are always in a parent category and have no further subs of themselves. Then two tables is ok.
If it's like a tree though, where there are just categories, which can both be children and have children, you should use one table (Google "nested set").
(Or maybe you don't mean category/subcategory but primary category/secondary category, where the secondary category is not fixed to a certain primary category. Electronics + Cycling instead of Cycling->Speedometers. Then you could use one table if it could also be Cycling + Electronics)

Answer 3

It is obviously work. But you may use separate tables.
create table categories
(
categoryId int not null,
categoryName varchar(20) not null,
primary key(categoryId)
);
create table subcategories
(
subcategoryId int not null,
subcategoryName varchar(20) not null,
parentId int not null,
primary key(subcategoryId),
foreign key(categoryId) references categories(categoryId)
);
create tables items
(
item_id int unsigned not null auto_increment,
name varchar(255) not null,
description varchar(100) not null,
primary key(item_id),
foreign key(categoryId) references categories(categoryId),
foreign key(subcategoryId) references subcategories(subcategoryId)
)