我不能用函数从表中发送数值

Alter function calcular(@cajas int, @peso int)
RETURNS int
AS
BEGIN
  set @cajas = (select cant_entr from entrega)
  set @peso = (select peso_entr from entrega)

  DECLARE @mercaderia int;
  SELECT @mercaderia = @cajas * @peso * 0.5

  RETURN @mercaderia;
END
go 

SELECT 
    dbo.calcular(cant_entr, peso_entr), matr_veh 
FROM
    entrega, vehiculo 
WHERE 
    matr_veh = 'PTK-0165'
Alter function calcular(@cajas int, @peso int)