CREATE TABLE invoices (
	some_data TEXT
	,customer_id INT
	,invoice_specific_payment_terms TEXT
);

INSERT INTO invoices
SELECT 'abc',1,'60 days from receipt of invoice';

REFRESH TABLE invoices;

--> NB in this example invoice_specific_payment_terms is never NULL

CREATE TABLE customers (
	customer_id INT
	,customer_default_payment_terms TEXT
);

INSERT INTO customers
SELECT 1,'30 days from receipt of invoice';

REFRESH TABLE customers;

CREATE TABLE distributed_summits AS
SELECT * FROM sys.summits;

ANALYZE;

SELECT some_data,
	CASE
		WHEN invoice_specific_payment_terms IS NOT NULL
			THEN invoice_specific_payment_terms
		ELSE (	SELECT customer_default_payment_terms
				FROM customers
				CROSS JOIN distributed_summits s1				
				CROSS JOIN distributed_summits s2
				CROSS JOIN distributed_summits s3
				WHERE customers.customer_id=invoices.customer_id
				ORDER BY s3.height DESC
				LIMIT 1
			)
	END 
FROM invoices;