Split customers into spend quartiles

Table orders:

Add up each customer's orders, then split customers into 4 groups of (nearly) equal size by total spend: quartile 1 holds the lowest spenders and quartile 4 the highest. When the customer count does not divide by 4, the first groups get the extra customers. Customers with equal totals are ordered by customer_id.

Output columns: customer_id, total, quartile.

CREATE TABLE orders (
  id int PRIMARY KEY,
  customer_id int NOT NULL,
  amount int NOT NULL
);