Select only one row from a join based on foreign condition - mysql

Question

The main idea
user hasMany roles (data is stored in the tables: users, roles, user_role)
i want to know if the user is admin OR client
this data will then be joined to a different result
What I'm doing
SELECT
`users`.`id`,
`users`.`name`,
`roles`.`display_name`
FROM `users`
JOIN role_user ON users.id = role_user.user_id
JOIN roles ON role_user.role_id = roles.id
Why it's wrong
Because this is the result I get
id Name Display name
1 admin Admin
1 admin Client
2 admin2 Admin
2 admin2 Client
3 client Client
7 test Admin
7 test Client
What I want
id Name Display name
1 admin Admin
2 admin2 Admin
3 client Client
7 test Admin
How I'm working to make this work
Using aggregates somehow
Using cases somehow
Joining with a subset of the data somehow
Thank you for your idea !
[Update] Here is an sqlFiddle describing the issue.

Answer 1

SELECT
`users`.`id`,
`users`.`name`,
`roles`.`display_name`
FROM `users`
LEFT JOIN role_user ON users.id = role_user.user_id
left JOIN roles ON role_user.role_id = roles.id

Answer 2

This solution should work fine for you:
select * from users
where users.id in (
select users.id FROM users
JOIN role_user ON (users.id = role_user.user_id)
JOIN roles ON (role_user.role_id = roles.id)
where roles.display_name like 'admin'
)
Union
select * from users
where users.id not in (
select users.id FROM users
JOIN role_user ON (users.id = role_user.user_id)
JOIN roles ON (role_user.role_id = roles.id)
where roles.display_name like 'admin'
);
Fiddle here: http://sqlfiddle.com/#!9/0fabba/42
I would recommend to create a view for the sub-query, but I couldn't do it on fiddle.
Example using a view:
Create or replace view admin_list as
select users.id FROM users
JOIN role_user ON (users.id = role_user.user_id)
JOIN roles ON (role_user.role_id = roles.id)
where roles.display_name like 'admin';
Then the query will be much compact:
select * from users
where users.id in (
select users.id FROM admin_list
)
Union
select * from users
where users.id not in (
select users.id FROM admin_list
);

Answer 3

My own solution was the following
create a view to get the user's role id (either admin or client)
join with the view
Specifically here is the view
CREATE VIEW vw_users_role_by_priority AS (
SELECT
role_user.user_id,
min(role_user.role_id) AS 'role_id'
FROM role_user
JOIN roles ON role_user.role_id = roles.id
GROUP BY role_user.user_id
);
And here is the final query
SELECT
`users`.`id`,
`roles`.display_name
FROM `users`
JOIN vw_users_role_by_priority ON vw_users_role_by_priority.user_id = users.id
JOIN roles ON roles.id = vw_users_role_by_priority.role_id
ORDER BY `users`.`name` ASC;

Answer 4

Try with this query :
SELECT users.`id`, users.`name`, MIN(roles.`display_name`)
FROM `users` users, `role_user` role_user, `roles` roles
WHERE users.id = role_user.user_id AND role_user.role_id = roles.id
GROUP BY users.`id`
In your query you join tables which give you all possible combinations of results. This query will select only the role_user/roles for each found userId.
EDIT : added "group by" to select only unique users and "min" to select its alphabetically lowest role (so "admin" get selected before "client")