Can this query be rewritten without UNION, and is it scaleable? - mysql

Question

I have a few tables:
In the product-table, I have a list of products.
In the user-table, I have a list of users.
In the group-table, I have groups of users.
IN the group_member-table, I have linked group and member (many-to-many)
In the user_product-table, I have linked user and product (many-to-many)
In the group_product-table, I have linked group and product (many-to-many)
So a user could have many products, a product could have many users. A user can be member of many groups, a group could have many members. A group could have many products, a product could have many groups. In other words, a product can have both groups and users.
What I want to ask the database is: "List all the products that a given user has access to, either through a direct relation in the user_product-table, or through the groups that the user is member of. I want the name of the product and the name of the user."
This is the query that I have come up with:
# First get all the products the user has access to via a group.
SELECT product.name,
user.first_name
FROM product
INNER JOIN group_product
ON group_product.product_id = product.product_id
INNER JOIN group
ON group.group_id = group_product.group_id
INNER JOIN group_member
ON group_member.group_id = group.group_id
INNER JOIN user
ON user.user_id = group_member.user_id
WHERE user.user_id = 1
UNION
# Now get all the products via direct access from user_product.
SELECT product.name,
user.first_name
FROM product
INNER JOIN user_product
ON user_product.product_id = product.product_id
INNER JOIN user
ON user.user_id = user_product.user_id
WHERE user.user_id = 1
Is this a good query, or is it better/possible to rewrite this into a JOIN only query? Would this be a fast query if there were 100 000 users, 10 000 groups and 100 products?
Is this a good database design, or is it better to store this logic in another manner?
(This is my first more complex query.)

Answer 1

Your query has the correct approach for your data model. The "correctness" of your data model really depends on volumes and frequency of change- you could opt to always store the explicit user-product relationship whenever a user is added to or removed from a group. This is a denormalizing tactic and moves the overhead from querying to updating - usually best not to consider these moves unless performance is tested and deficient.
A very tiny optimisation may be to avoid the join to user and product until after the union. At present you are only selecting the product name and user first_name, but if you were selecting many columns the sort/distinct would involve more work than strictly necessary, so something like:-
select product.name, user.first_name
from
(
select
group_product.product_id
from
group_product
inner join group on group.group_id = group_product.group_id
inner join group_member on group_member.group_id = group.group_id
where group_member.user_id = 1
union
select product_id product.name,
from user_product
where user_product.user_id = 1
) as d
inner join product on product.product_id = d.product_id
inner join user on user.user_id = 1