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N021-M1 Tier 2 · Core SQL · medium hr · Helix Systems

Return the employee name and manager name for every Engineering team member who has a manager on record

Part of Self-Joins in SQL

The problem

An Engineering team lead at Helix Systems wants a report showing every Engineering employee alongside their direct manager's name.

Write a query to return the employee name and manager name for every Engineering team member who has a manager on record.

Assumptions:

  • The employees table contains every active and former employee at Helix Systems.
  • Engineering is identified by department_id = 1.
  • The department condition applies to the employee role only, not to the manager role.

Output:

  • One row per qualifying Engineering employee, with columns employee_name and manager_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
FROM
  employees e
  JOIN employees m ON e.manager_id = m.id
WHERE
  e.department_id = 1

The shape

The department filter applies to the employee role only, so the WHERE predicate names the employee alias (e.department_id = 1) and leaves the manager alias unconstrained. Managers can sit in any department; only the employee has to be in Engineering.

Clause by clause

  • SELECT e.name AS employee_name, m.name AS manager_name returns the employee name and manager name from the two aliased instances of employees.
  • FROM employees e reads the table in the employee role.
  • JOIN employees m ON e.manager_id = m.id reads the table again in the manager role and pairs each employee with their manager. Anyone with no manager_id is dropped at this step.
  • WHERE e.department_id = 1 restricts the employee role to Engineering. Marcus Reid (Engineering, managed by Sarah Chen from a different department) is in the result; Sarah Chen as a manager of Marcus is reachable through m regardless of her own department.

Why this and not WHERE m.department_id = 1

The filter has to name the role it constrains. e.department_id = 1 keeps every Engineering employee paired with whoever their manager happens to be. m.department_id = 1 would do the opposite — keep every employee whose manager is in Engineering, regardless of the employee's own department. Same column on a different alias, different question. The prompt asks for Engineering employees, so the filter goes on e.

The trap

When both aliases share a column, the alias prefix on the WHERE filter is what scopes the result. Writing WHERE department_id = 1 without a prefix is a query error (the reference is ambiguous), but the more silent failure is writing the right column on the wrong alias and getting a plausible-looking result for the wrong question.

You practiced restricting one role of a self-join independently. Each alias can carry its own WHERE predicates — the employee condition and the manager condition stay separate.

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