Question

It is pretty straight forward, but I could not find an answer. If the combination does exist, then UPDATE, else INSERT.

IF EXISTS(SELECT customer_id FROM payment_conditions WHERE customer_id = 2 AND shipping_company_id = 1) 

THEN
UPDATE payment_conditions SET payment_condition = "pay in full" WHERE customer_id = 2  AND shipping_company_id = 1;

ELSE
INSERT INTO payment_conditions customer_id, shipping_company_id, payment_condition, active VALUES (2,1,"some value",1);


END IF

Answer 1

You should set customer_id and shipping_company_id to be foreign keys, so you can use

INSERT INTO payment_conditions
    customer_id, shipping_company_id, payment_condition, active
VALUES (2, 1, "some value", 1)
ON DUPLICATE KEY UPDATE payment_condition = "pay in full"