Join two of the same tables to another table and output the info of the (same) table in the same row - sql

Question

Sorry for the bad/long title but I don't know how else to put it.
What I want to do is join to 'A' tables and join it to the 'B' table where both 'A' have a foreign key in common and display info from both 'A' tables in the same row while preventing duplicates such as the example in the pic:
I know the query is just doing it's job, but is there a way to prevent 'duplicates' by comparing between the rows before output?
Here's what I tried, I know it may be bad performance-wise and there may be better ways but this is for a mini-project with a small DB, where performance shouldn't really matter:
SELECT w.emp_id AS emp1_id, w2.emp_id AS emp2_id,
e.fname || ' ' || e.lname AS emp1_name, e1.fname || ' ' || e1.lname AS emp2_name,
e.jobtitle AS emp1_jobtitle, e1.jobtitle AS emp2_jobtitle, e2.fname || ' ' || e2.lname AS cs_name
FROM work_on w
LEFT JOIN work_on w2
on w.emp_id != w2.emp_id and w.ticket_id = w2.ticket_id
LEFT JOIN employee e
on w.emp_id = e.emp_id
LEFT JOIN employee e1
on w2.emp_id = e1.emp_id
LEFT JOIN ticket t
on t.ticket_id = w.ticket_id
LEFT JOIN customer_problem p
on p.problem_id = t.problem_id
LEFT JOIN employee e2
on e2.emp_id = p.emp_id
WHERE e2.emp_id = 20 and p.submit_date >= '2018-04-08'
and p.submit_date <= '2018-04-11' and e1.emp_id != e.emp_id
ORDER BY w.emp_id;
My tables:
Employee: | Work_On: | Ticket: | Problem
----------+------------+--------------+------------
emp_id work_id ticket_id problem_id
fname emp_id problem_id emp_id
lname ticket_id
In this case I'm trying to combine two Employee on Work_On where they have the Ticket in common and another Employee which connects to the ticket via the Problem table.

Answer 1

Here is one option using least/greatest:
SELECT DISTINCT
LEAST(w.emp_id, w2.emp_id) AS emp1_id,
GREATEST(w.emp_id, w2.emp_id) AS emp2_id,
LEAST(e.fname || ' ' || e.lname, e1.fname || ' ' || e1.lname) AS emp1_name,
GREATEST(e.fname || ' ' || e.lname, e1.fname || ' ' || e1.lname) AS emp2_name,
LEAST(e.jobtitle, e1.jobtitle) AS emp1_jobtitle,
GREATEST(e.jobtitle, e1.jobtitle) AS emp2_jobtitle,
e2.fname || ' ' || e2.lname AS cs_name
FROM work_on w
LEFT JOIN work_on w2
ON w.emp_id != w2.emp_id AND w.ticket_id = w2.ticket_id
LEFT JOIN employee e
ON w.emp_id = e.emp_id
LEFT JOIN employee e1
ON w2.emp_id = e1.emp_id
LEFT JOIN ticket t
ON t.ticket_id = w.ticket_id
LEFT JOIN customer_problem p
ON p.problem_id = t.problem_id
LEFT JOIN employee e2
ON e2.emp_id = p.emp_id
WHERE
e2.emp_id = 20 AND
p.submit_date >= '2018-04-08' AND
p.submit_date <= '2018-04-11' AND
e1.emp_id != e.emp_id
ORDER BY w.emp_id;
To see why the least/greatest trick works, consider the following two records/columns:
emp1_id | emp2_id
2 | 15
15 | 2
It should be clear that while these records are distinct now, if we instead choose the least id followed by the greatest id, they appear identical:
LEAST(emp_id1, emp_id2) | GREATEST(emp_id1, emp_id2)
2 | 15
2 | 15
Then, using SELECT DISTINCT removes one of the two duplicate rows.