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N021-M3 Tier 2 · Core SQL · medium hr · Helix Systems

Return all three values for every employee with a manager on record

Part of Self-Joins in SQL

The problem

Helix Systems' HR reporting team needs a table showing each employee's name, their manager's name, and the department the employee belongs to.

Write a query to return all three values for every employee with a manager on record.

Assumptions:

  • The employees table contains every active and former employee at Helix Systems.
  • The departments table contains every department; each employee's department_id points to a row in departments.
  • The query brings together two relationships: each employee paired with their manager (both from employees), and each employee paired with their department.

Output:

  • One row per employee with a manager, with columns employee_name, manager_name, and department_name.
Schema · hr 4 tables
departments
id integer
name text
location text
budget numeric
salaries
id integer
employee_id integer
amount numeric
effective_date date
end_date? date
employees
id integer
name text
email text
department_id integer
manager_id? integer
hire_date date
title text
is_active boolean
job_history
id integer
employee_id integer
title text
department_id integer
start_date date
end_date? date
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Solution query
SELECT
  e.name AS employee_name,
  m.name AS manager_name,
  d.name AS department_name
FROM
  employees e
  JOIN employees m ON e.manager_id = m.id
  JOIN departments d ON e.department_id = d.id

The shape

Three aliases name three roles in the query: e is the employee, m is the manager (also from employees), and d is the department from departments. The self-join supplies the manager name; a second, ordinary join supplies the department name.

Clause by clause

  • SELECT e.name AS employee_name, m.name AS manager_name, d.name AS department_name pulls name from each of the three aliases. Three different roles, three labeled columns.
  • FROM employees e reads employees in the employee role.
  • JOIN employees m ON e.manager_id = m.id reads employees again in the manager role. This is the self-join; both sides reference the same physical table, distinguished only by the aliases.
  • JOIN departments d ON e.department_id = d.id reads departments and links each employee to their department. The join is on the employee's department_id, not the manager's — the prompt asks for the department the employee belongs to.

Why this and not JOIN departments d ON m.department_id = d.id

Which alias the department join keys to decides whose department appears in the result. e.department_id = d.id pairs each row with the employee's department (Marcus Reid → Engineering). m.department_id = d.id would pair each row with the manager's department instead (Marcus Reid → whatever department Sarah Chen sits in). The prompt asks for the employee's department, so the join keys to e.

The trap

Once two aliases share a foreign key column (department_id), every downstream join has to commit to one of them. The query parses either way and returns the same column count and similar row count, so the mistake doesn't surface as an error. Trace which role the prompt is asking about before writing the second join's ON clause.

You practiced combining a self-join with a regular join. Aliases keep the roles distinct — e, m, and d each name a different participant, even when two of them come from the same table.

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