How do you return parent rows that have at least N children?

Short answer: JOIN children, GROUP BY parent key, HAVING COUNT(*) >= N.

Sample solution

SELECT d.DepartmentId, d.Name, COUNT(*) AS EmpCount
FROM Departments d
INNER JOIN Employees e ON e.DepartmentId = d.DepartmentId
GROUP BY d.DepartmentId, d.Name
HAVING COUNT(*) >= 5;

INNER JOIN excludes departments with zero employees — use LEFT JOIN if zeros matter.