Query Postgres
like you mean it.

-- Top customers by revenue, last 30 days
SELECT
  c.id,
  c.name,
  SUM(il.amount_cents)::numeric / 100 AS revenue
FROM invoices i
JOIN invoice_lines il ON il.invoice_id = i.id
JOIN customers   c  ON c.id = i.customer_id
WHERE i.paid_at >= now() - interval '30 days'
GROUP BY c.id, c.name
ORDER BY revenue DESC
LIMIT 10;