MediumSQL challenge #124JOIN

List Employees with Their Manager Names Using a Self Join

Return each employee's first name next to the first name of their manager.

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

Show employees with their manager name.

A correct answer returns 120 rows with 2 columns: first_name, manager.

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	manager
William	NULL
John	NULL
Daniel	NULL
Melissa	NULL
Jason	NULL

First 5 of 120 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
Keeping every row of the left table with LEFT JOIN, matched or not
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 e.
LEFT JOIN employees again, under the alias m, on e.manager_id = m.id. Rows from the left side that have no match are kept, with NULL in the joined columns.
Return only the columns e.first_name and m.first_name.

Model solution

Checking your session…

What to watch for

The employees table is joined to itself under two aliases, one playing the employee and one the manager. A LEFT JOIN keeps the ten people at the top who have no manager; an inner join would drop them.

Self join. Without a condition such as a.id < b.id, a self join pairs every row with itself and lists each pair twice.
LEFT JOIN. A WHERE condition on the right-hand table removes the unmatched rows and turns the LEFT JOIN back into an inner join. Conditions on that table go in the ON clause.