GROUP BY and HAVING — managers with at least 2 people

I group employees by their manager and filter the already-counted groups.

-- employees(id, name, manager_id)
SELECT manager_id, COUNT(*) AS team_size
FROM employees
WHERE manager_id IS NOT NULL
GROUP BY manager_id
HAVING COUNT(*) >= 2;

To be precise: WHERE removes rows before aggregation, HAVING filters its results. These are direct reports, not everyone below in the hierarchy.

Pitfall: a later JOIN to a 1:N table can multiply the employees and inflate the count.