SELECT campo1, campo2, campo3/campo4 AS nomeQueQuiserDar FROM tabela

CREATE VIEW nome_view AS
SELECT campo1, campo2, campo3/campo4 AS divisao FROM tabela;

SELECT campo1, campo2, divisao FROM nome_view

CREATE TRIGGER nome_trigger AFTER INSERT
ON tabela
FOR EACH ROW UPDATE tabela SET NEW.campo3 = NEW.campo1 / NEW.campo2