Write a query to list departments that have more than 5 employees.

Count heads in each department, then keep only the departments whose count passes the bar. Counting must happen first, so the filter on the count goes in HAVING.

Table employees(id, name, dept_id).

SELECT dept_id, COUNT(*) AS headcount
FROM employees
GROUP BY dept_id
HAVING COUNT(*) > 5;

• GROUP BY dept_id forms one group per department.
• COUNT(*) counts the rows in each group.
• HAVING COUNT(*) > 5 removes groups of 5 or fewer.

To show the department name, join to departments and group by the name as well:

SELECT d.name, COUNT(*) AS headcount
FROM employees e
JOIN departments d ON d.id = e.dept_id
GROUP BY d.id, d.name
HAVING COUNT(*) > 5;

An employee with a NULL dept_id would form a NULL group in the first query.

Time: O(n) with hash aggregation, or O(n log n) if the database sorts to group. Space: O(g) for g groups.