How to "remove duplicates" from a UNION query - mysql

Question

I have two tables in MySQL: One called gtfsws_users which contains users for a system I'm developing and another called gtfsws_repository_users which contains roles for these users.
gtfsws_users has these fields: email, password, name, is_admin and enabled.
gtfsws_repository_users has: user_email, repository_id and role.
The role is an integer that defines privileges over a GTFS repository (public transportation data, not relevant for my problem).
One important thing is that every administrator accont (that is, every user that has the is_admin flag set as 1 in gtfsws_users) has full access to all repositories.
Now, only users registered in gtfsws_repository_users will have access to a specific repository defined there (unless they are administrators, of course). One user can have multiple repositories which him/her can access.
What I'm trying to do is to get all users with access to a specific repository (it doesn't matter which type of role the user has, I just want to know if they can access the repository or not). So I'm writing this SQL statement for getting them:
(
SELECT DISTINCT
gtfsws_users.email AS email,
gtfsws_users.name AS name,
gtfsws_users.is_admin AS is_admin,
gtfsws_users.enabled AS enabled,
gtfsws_repository_users.role AS role
FROM
gtfsws_users
INNER JOIN
gtfsws_repository_users
ON
gtfsws_users.email = gtfsws_repository_users.user_email
WHERE
gtfsws_repository_users.repository_id = '2'
)
UNION
(
SELECT
email,
name,
is_admin,
enabled,
null AS role
FROM
gtfsws_users
WHERE
is_admin = 1
)
Now, this works fine for users with access to different repositories. It also gets all administrators too.
The problem is when I have an administrator that is also registered in gtfsws_repository_users, because I get it duplicated.
So for example, it I have this in gtfsws_users:
('test#test.com', '*****', 'Real name', 1, 1)
And also the user is registered in gtfsws_repository_users as this:
('test#test.com', 2, 10)
When I do the SELECT in MySQL (using the UNION to add all administrators) I get:
('test#test.com', 'Real name', 1, 1, 10)
('test#test.com', 'Real name', 1, 1, NULL)
What I need to do is to filter that table to remove duplicates, that is getting only:
('test#test.com', 'Real name', 1, 1, NULL)
Yes, getting NULL as the role (since it will be ignored as the user is an administrator).
Does anybody have a clue on how to achieve that?
Thanks a lot.
EDIT: Ok, thanks to Katrin's suggestion, I'm getting some progress. I do get one row, but it's the one with the role number defined. Any way to preserve the one with the NULL role instead of the defined one?

Answer 1

Since aggregate functions ignore null values, you can convert null to a number that can be extracted as min or max.
Assuming all your roles are greater than 0:
SELECT email, name, is_admin, enabled, nullif(min(coalesce(role, 0)), 0) as role
from
((
SELECT DISTINCT
gtfsws_users.email AS email,
gtfsws_users.name AS name,
gtfsws_users.is_admin AS is_admin,
gtfsws_users.enabled AS enabled,
gtfsws_repository_users.role AS role
FROM
gtfsws_users
INNER JOIN
gtfsws_repository_users
ON
gtfsws_users.email = gtfsws_repository_users.user_email
WHERE
gtfsws_repository_users.repository_id = '2'
)
UNION
(
SELECT
email,
name,
is_admin,
enabled,
null AS role
FROM
gtfsws_users
WHERE
is_admin = 1
)) as Q
GROUP BY email, name, is_admin, enabled