Is there a database can represent set-like structure? - mongodb

Question

I mainly focus on the query operation, not union or intersection.
Here is an example.
Let say we have a multi-level category:
CATEGORY-TOP-LEVEL:
CATEGORY1:
CATEGORY1.1:
item1
CATEGORY2:
CATEGORY2.1:
item2
Here, item[N] is the data. Category is a tree structure to represent which category the item belongs to.
Now, suppose I'd like to query all data in category 1, the database should give me item1.
Suppose I'd like to query all data in category-top-level, the database should give me item1 and item2.
It's like set theory. Because item1 belongs to CATEGORY1.1, and CATEGORY1.1 belongs to CATEGORY1. Thus item1 belongs to CATEGORY1.
One solution is use Materialized Paths: We put an field in item, named path, the value is like: ",CATEGORY-TOP-LEVEL,CATEGORY1,CATEGORY1.2". But the problem is it will cause a lot of writing operations when I change a category's name or the hierarchy of the category.
Can MongoDB support that? if not, is there a database can support that?
P.S. Let's take query performance into consideration.

Answer 1

Every modern relational database can support that.
There are different ways of modeling this in a relational database, the most common one is called the "adjacency model":
create table category
(
id integer primary key,
name varchar(100) not null,
parent_category_id integer references category
);
If an item can only ever belong to a single category, the item table would look like this:
create table item
(
id integer primary key,
name varchar(100) not null,
category_id integer not null rerences category
);
If an item can belong to more then one category, you need a many-to-many relationship (also very common in the relational world)
To get all categories below a certain category you can use a recursive query:
with recursive cat_tree as (
select id, name, parent_category_id
from category
where id = 42 -- this is the category where you want to start
union all
select c.id, c.name, c.parent_category_id
from category c
join cat_tree p on p.id = c.parent_category_id
)
select *
from cat_tree;
To get the items together with the categories, just join the above to the item table.
The above query is standard ANSI SQL.
Other popular models are the nested set model, the materialized path (you mentioned that) and the closure table.
This gets asked a lot. See the tags recursive-query and hierarchical-data for many more examples.