Who are your top 20 customers, and how much are they worth?

SELECT
  c.company_name,
  c.account_manager,
  COUNT(DISTINCT o.id)                          AS order_count,
  ROUND(SUM(o.total_amount)::numeric, 2)        AS total_revenue,
  ROUND(SUM(o.total_amount)::numeric
    / NULLIF(COUNT(DISTINCT o.id), 0), 2)       AS avg_deal_size,
  MIN(o.order_date)::date                       AS first_order,
  MAX(o.order_date)::date                       AS last_order,
  ROUND(SUM(o.total_amount)::numeric * 100
    / SUM(SUM(o.total_amount)) OVER ()::numeric, 2) AS pct_of_total_revenue
FROM orders o
JOIN customers c ON c.id = o.customer_id
WHERE o.order_date >= DATE_TRUNC('year', CURRENT_DATE)
  AND o.status NOT IN ('cancelled', 'refunded')
GROUP BY c.id, c.company_name, c.account_manager
ORDER BY total_revenue DESC
LIMIT 20