DB2 SQL - Select all values of a table related to another table - sql

Question

Maybe I'm not clear on the topic for what I'm trying to ask, so here is my question:
I have three tables for now, say, User, User Role, Role, which make reference to each other:
User
========
USER_ID | USER_NAME
1 | User A
2 | User B
User Role
========
USER_ID | ROLE_ID
1 | 1
1 | 2
2 | 1
Role
========
ROLE_ID | ROLE_NAME
1 | Admin
2 | Supervisor
As you can see, User A has two role for now: Admin and Supervisor
What I'm trying to do now is selecting all users with role Admin, then the sql result should display all other roles related to that user also.
Say, the sql is executed like this:
select A.*, C.ROLE_NAME from User A, User Role B, Role C where A.USER_ID = B.USER_ID and B.ROLE_ID = C.ROLE_ID and B.ROLE_ID = '1'
The above sql should only display User A with only one role, Admin:
USER_ID | USER_NAME | ROLE_NAME
1 | User A | Admin
2 | User B | Admin
But what I want is, beside Admin, the sql result should also display User A with ROLE_NAME = Supervisor:
USER_ID | USER_NAME | ROLE_NAME
1 | User A | Admin
1 | User A | Supervisor
2 | User B | Admin
Any ideas?

Answer 1

Use exists:
with ur as (
select u.*, C.ROLE_NAME
from User u join
User_Role ur
on u.user_id = ur.user_id join
Role r
on r.role_id = ur.role_id
)
select ur.*
from ur
where exists (select 1
from ur ur2
where ur2.user_id = ur.user_id and ur2.role_name = 'admin'
);
Also, never use commas in the FROM clause. Always use proper, explicit JOIN syntax with the condition in the ON clause.
Second, use meaningful table aliases rather than arbitrary letters such as "a", "b", and "c".