SQL:create table column, "as constant value" :   create table factura (   
    importe money,
	unidades_vendidas int,
    subtotal as (unidades_vendidas * importe),
	total  as (subtotal * 1.18), -- (1.18 needs to be a constant value)
)

how to define "total" to be as a 1.18 value of "subtotal"