DB2 SQL-选择与另一个表相关的表的所有值

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".