Colleague pairs in the same city, different teams

The people team is planning cross-team coffee meetups and wants to pair colleagues who work in the same city but belong to different departments.

From staff, return each pair once as staff_a, dept_a, staff_b, dept_b and city. A pair must not appear twice in mirrored order, and nobody may be paired with themselves. Order by city, then the two staff_id values.

1. Step 1: Join the table to itself with two aliases, `a` and `b`, so each row can be compared with every other row.
2. Step 2: `a.city = b.city` keeps same-city pairs and `a.department <> b.department` keeps different departments.
3. Step 3: `a.staff_id < b.staff_id` removes self-pairs and keeps only one of each mirrored pair (A,B) and (B,A).

SELECT a.staff_name  AS staff_a,
       a.department  AS dept_a,
       b.staff_name  AS staff_b,
       b.department  AS dept_b,
       a.city
FROM staff a
JOIN staff b
  ON a.city = b.city
 AND a.department <> b.department
 AND a.staff_id < b.staff_id
ORDER BY a.city, a.staff_id, b.staff_id;