How work with SQL and multiple join or merge them - mariadb

Question

I need to retrieve events from table, with skills requirements and user engagements and skills.
For a project, I have :
an users table
a skills table with inheritance from others skills
an users_skills table with skills of users
an events table
an events_skills table with the quantity of a required skills for the events
an events_users table with users engagements on events
CREATE TABLE `users` (
`id` int(11) unsigned NOT NULL AUTO_INCREMENT PRIMARY KEY,
`name` char(32) NOT NULL
);
CREATE TABLE `skills` (
`id` int(11) unsigned NOT NULL AUTO_INCREMENT PRIMARY KEY,
`name` char(64) NOT NULL,
`parent_id` int(11) unsigned DEFAULT NULL,
FOREIGN KEY (`parent_id`) REFERENCES `skills` (`id`)
);
CREATE TABLE `users_skills` (
`id` int(11) unsigned NOT NULL AUTO_INCREMENT PRIMARY KEY,
`user_id` int(11) unsigned NOT NULL,
`skill_id` int(11) unsigned NOT NULL,
FOREIGN KEY (`user_id`) REFERENCES `users` (`id`),
FOREIGN KEY (`skill_id`) REFERENCES `skills` (`id`)
);
CREATE TABLE `events` (
`id` int(11) unsigned NOT NULL AUTO_INCREMENT PRIMARY KEY,
`name` char(64) NOT NULL
);
CREATE TABLE `events_skills` (
`id` int(11) unsigned NOT NULL AUTO_INCREMENT PRIMARY KEY,
`event_id` int(11) unsigned NOT NULL,
`skill_id` int(11) unsigned NOT NULL,
`quantity` smallint unsigned NOT NULL,
FOREIGN KEY (`event_id`) REFERENCES `events` (`id`),
FOREIGN KEY (`skill_id`) REFERENCES `skills` (`id`)
);
CREATE TABLE `events_users` (
`id` int(11) unsigned NOT NULL AUTO_INCREMENT PRIMARY KEY,
`event_id` int(11) unsigned NOT NULL,
`user_id` int(11) unsigned NOT NULL,
FOREIGN KEY (`event_id`) REFERENCES `events` (`id`),
FOREIGN KEY (`user_id`) REFERENCES `users` (`id`)
);
I want to retrieve all events, with skills requirments and users participation.
My first idea was to use LEFT JOIN :
SELECT e.id, e.name, es.skill_id, s.name skill_name, es.quantity,
eu.user_id, u.name user_name, us.skill_id user_skill_id,
uss.name user_skill_name
FROM events e
LEFT JOIN events_skills es ON es.event_id = e.id
LEFT JOIN skills s ON s.id = es.skill_id
LEFT JOIN events_users eu ON eu.event_id = e.id
LEFT JOIN users u ON u.id = eu.user_id
LEFT JOIN users_skills us ON us.user_id = u.id
LEFT JOIN skills uss ON uss.id = us.skill_id;
id name skill_id skill_name quantity user_id user_name user_skill_id user_skill_name
1 eve1 2 ski2 3 1 use1 2 ski2
1 eve1 3 ski2-2 1 1 use1 2 ski2
1 eve1 2 ski2 3 2 use2 NULL NULL
1 eve1 3 ski2-2 1 2 use2 NULL NULL
2 eve2 NULL NULL NULL NULL NULL NULL NULL
2 differents events, second without users and skills. First, with 2 users, first with one skills, seconds without skills, and finally, two skills requirements.
It work "fine", but it's verry ugly, and I think it can be very slow if an event or an user has too many skills.
Also, each event has a lot of lines, and it's difficult to sort them (if I want to display only 20 events...).
To improve, I think I can load all skills separately and remove the two LEFT JOIN for table skills.
But, can I merge all skills and quantity in one column, and users in others ? Did you have betters solutions/improvements ?
id name skills users
1 eve1 2,3;3,1 1,2;2,NULL
2 eve2 NULL NULL
Thanks for your help.

Answer 1

Another way to do what Thorsten suggested:
select e.*,
( SELECT group_concat(skill_id order by skill_id separator ',')
from events_skills
WHERE event_id = e.id
) AS skills,
( SELECT group_concat(user_id order by user_id separator ',' )
from events_users
WHERE event_id = e.id
) AS users
from events e
order by e.id;
Each subquery needs an extra join to get the "name" of the skill and user instead of the "id".
May I suggest you use the name of the skill as its id.
When you have a many-many mapping table, be sure to use the pair of ids as the PRIMARY KEY and have an INDEX in the opposite order. And don't have an AUTO_INCREMENT. Details: http://mysql.rjweb.org/doc.php/index_cookbook_mysql#many_to_many_mapping_table

Answer 2

A result with skills and users as comma-separated strings is exactly how I would do this, too. Use GROUP_CONCAT for this.
select e.*, s.skills, u.users
from events e
left join
(
select event_id, group_concat(skill_id order by skill_id separator ',') as skills
from events_skills
group by event_id
) s on s.event_id = e.id
left join
(
select event_id, group_concat(user_id order by user_id separator ',') as users
from events_users
group by event_id
) u on u.event_id = e.id
order by e.id;