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N020-M3 Tier 2 · Core SQL · medium hr · Helix Systems

Return the employee name and department name for every Engineering-employee × department combination

Part of CROSS JOIN in SQL

The problem

A training coordinator at Helix Systems needs every possible pairing of an Engineering-department employee (department_id = 1) with every company department for a cross-functional assignment matrix.

Write a query to return the employee name and department name for every Engineering-employee × department combination.

Assumptions:

  • The employees table contains every active and former employee at Helix Systems.
  • The departments table contains every department; Engineering is department_id = 1.
  • Each Engineering employee should appear once paired with every department, including Engineering itself.

Output:

  • One row per Engineering-employee × department combination, with columns employee_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,
  d.name AS department_name
FROM
  employees e
  CROSS JOIN departments d
WHERE
  e.department_id = 1

The shape

WHERE e.department_id = 1 narrows employees to the Engineering cohort, and CROSS JOIN departments d pairs each Engineering employee with every department in the company — including Engineering itself. The result is the cross-functional assignment matrix the training coordinator needs.

Clause by clause

  • FROM employees e CROSS JOIN departments d is the unconditional pairing. Every row in employees combines with every row in departments.
  • WHERE e.department_id = 1 restricts the left side to Engineering employees. The integer 1 is the literal value of department_id for Engineering; the filter reads the employee's own department from the employees table, not from the joined departments row.
  • e.name AS employee_name pulls the employee's name from the left side. d.name AS department_name pulls the department name from the right. Both source tables have a name column, so the table aliases are what tell PostgreSQL which one each reference resolves to.

Why this and not INNER JOIN ... ON e.department_id = d.id

An INNER JOIN on department_id would return one row per Engineering employee paired with the Engineering department only — the existing assignment, nothing more. The matrix the coordinator is building needs every Engineering employee paired with every department, including the ones they could rotate into. CROSS JOIN produces that full grid by not matching on the department relationship at all. The Engineering filter is on the employee side; the department side is left wide open.

You practiced a CROSS JOIN where one side has been pre-filtered to a cohort. The shape generalises: any time one side is a cohort and the other is a complete catalogue, WHERE cohort_filter + CROSS JOIN produces the cohort-by-catalogue grid.

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