MediumSQL challenge #146JOIN

Find Pairs of Employees in the Same Department

Return every pair of employees who work in the same department.

The challenge runs on the SQLabHub practice database (SQLite) and uses the employees table. You can solve it in the browser, with nothing to install.

Solve This SQL Challenge

The task

Find pairs of employees in same department.

A correct answer returns 683 rows with 2 columns: first_name, first_name.

Tables used in this challenge

`employees` 120 rows

id INTEGER (PK) · first_name TEXT · last_name TEXT · email TEXT · department TEXT · department_id INTEGER · title TEXT · salary REAL · hire_date TEXT · manager_id INTEGER · city TEXT · status TEXT

First 3 rows of the employees table
id	first_name	last_name	email	department	department_id	title	salary	hire_date	manager_id	city	status
1	William	Lewis	wlewis18@corp.io	Customer Success	7	Senior CS Manager	61774	2022-07-24	NULL	San Diego	active
2	John	Miller	jmiller51@corp.io	Legal	8	Paralegal	191694	2021-08-09	NULL	San Diego	active
3	Daniel	Rivera	drivera50@corp.io	Engineering	1	VP Engineering	135067	2016-06-06	NULL	Houston	active

The first rows of each table are shown. See how all the tables relate on the database schema page.

Expected output

Expected output of the challenge
first_name	first_name
William	Amy
William	Barbara
William	Charles
William	Dorothy
William	Edward

First 5 of 683 rows. This is the output of the model solution on the practice database. The task does not fix a sort order, so your rows may come back in a different order.

What you practise

Choosing exactly which columns a query returns
Combining rows from two tables on a matching key with an inner join
Joining a table to itself under two aliases

Hint and approach

Try the challenge first. Open these only when you need them.

Show the hint

Self join

Show the step-by-step approach

How the model solution is built, in the order the database evaluates it:

Read from the employees table, under the alias e1.
Join employees again, under the alias e2, on e1.department = e2.department, and only where e1.id is less than e2.id.
Return only the columns e1.first_name and e2.first_name.

Model solution

Checking your session…

What to watch for

The condition e1.id < e2.id does two jobs: it stops an employee being paired with themselves and it keeps only one of (A, B) and (B, A). Using <> instead would list each pair twice.

Self join. Without a condition such as a.id < b.id, a self join pairs every row with itself and lists each pair twice.
INNER JOIN. An inner join drops rows that have no partner on the other side, and it repeats a row once for every match, which inflates SUM and COUNT.