List the top 3 departments by revenue.

To find the top three departments by revenue, join the visits and billing tables on visit_id, aggregate the amount per department, order by the sum descending, and limit the result to three rows. Example:

SELECT v.department,

   SUM(b.amount) AS revenue

FROM visits v

JOIN billing b ON v.visit_id = b.visit_id

GROUP BY v.department

ORDER BY revenue DESC

LIMIT 3;

This query returns each department name and its total revenue, sorted from highest to lowest, showing only the top three.