Be aware of computed columns that use complex formula or call functions, they may lead to performance problems:

create function getsum(@id int)

returns int

as

begin

return (select sum(id) from sysobjects where id < @id)

end

GO

create table a(id int identity(1,1) primary key,a int,sum_id as dbo.getsum(id))

insert a select id from sysobjects

select * from a -- the function getsum is called for each row to compute sum_id

In this case, I prefer to use a trigger to compute the column.