Oracle Query on 3 tables with 2 outer joins - oracle11g

Question

I'm having some trouble writing a query that seems like it should be simple, but the solution is evading me.
We have three tables (simplified for the purpose of this question):
persons - a table of user names:
per_id number(10) - primary key, populated by a sequence
user_name varchar2(50)
user_id varchar2(15) - unique, basically the employee ID
work_assignments - kind of like crew assignments, but more general:
wa_id number(10) - primary key, populated by a sequence
wa_name varchar2(25)
current_assignments - which users have which work_assignments; the average per user is about 25 work assignments, but some "lucky" individuals have upwards of 150:
wa_id number(10)
per_id number(10)
I'm trying to write a query that will compare the work_assignments for two users, in a total of three columns. The results should look like this:
WA_Name User_Name1 User_Name2
Crew A Bob Joe
Crew B Joe
Crew C Bob
Basically, every work_assignment that either of the two user has, with the name(s) of the user(s) who has it.
Here's the closest I could come up with (well, I did come up with an ugly query with 3 subqueries that does the job, but it seems like there should be a more elegant solution):
select distinct * from (
select wa.name work_assignment,
per.name user_name1,
per2.name user_name2
from work_assignments wa join current_assignments ca on wa.wa_id = ca.wa_id
join current_assignments ca2 on wa.wa_id = ca2.wa_id
left outer join persons per on per.per_id = ca.per_id and per.user_id = 'X12345'
left outer join persons per2 on per2.per_id = ca2.per_id and per2.user_id = 'Y67890'
)
where user_name1 is not null or user_name2 is not null
order by 1;
The problem with this one is that if both users have a work assignment, it shows 3 records: one for Bob, one for Joe, and one for both:
WA_Name User_Name1 User_Name2
Crew A Bob Joe
Crew A Joe
Crew A Bob
Please help!
Thanks,
Dan

Answer 1

I created a set of sample data/tables
drop table persons;
drop table work_assgn;
drop table curr_assgn;
create table persons(
per_id number(10) not null
, user_name varchar2(10) not null
, user_id varchar2(10) not null
)
;
insert into persons values( 1, 'Bob', 'X123' );
insert into persons values( 2, 'Joe', 'Y456' );
insert into persons values( 3, 'Mike', 'Z789' );
insert into persons values( 4, 'Jeff', 'J987' );
commit;
create table work_assgn(
wa_id number(10) not null
, wa_name varchar2(25)
)
;
insert into work_assgn values( 10, 'Crew A' );
insert into work_assgn values( 20, 'Crew B' );
insert into work_assgn values( 30, 'Crew C' );
insert into work_assgn values( 40, 'Crew D' );
commit;
create table curr_assgn(
wa_id number(10) not null
, per_id number(10) not null
)
;
insert into curr_assgn values( 10, 1 );
insert into curr_assgn values( 10, 2 );
insert into curr_assgn values( 20, 2 );
insert into curr_assgn values( 30, 1 );
insert into curr_assgn values( 40, 4 );
commit;
select * from persons;
select * from work_assgn;
select * from curr_assgn;
So the data looks like
PERSONS
PER_ID USER_NAME USER_ID
---------- ---------- ----------
1 Bob X123
2 Joe Y456
3 Mike Z789
4 Jeff J987
WORK_ASSGN
WA_ID WA_NAME
---------- -------------------------
10 Crew A
20 Crew B
30 Crew C
40 Crew D
CURRASSGN
WA_ID PER_ID
---------- ----------
10 1
10 2
20 2
30 1
40 4
One approach may be to use a PIVOT
with assignment as
(
select p.user_id, p.user_name, a.wa_name
from persons p
join curr_assgn c
on p.per_id =c.per_id
join work_assgn a
on a.wa_id = c.wa_id
where p.user_id in ( 'X123', 'Y456' )
)
select * from assignment
pivot
( max(user_name) for user_id in ( 'X123', 'Y456' )
)
;