Question

I have a computed column called Cost that returns money.

I want to have another column that returns (Cost * 2), but it doesn’t allow me.

Answer 1

Computed columns cannot reference other computed columns. Though you ought to be able to just repeat the expression you would like to reference. From MSDN:

A computed column is computed from an expression that can use other columns in the same table. The expression can be a noncomputed column name, constant, function, and any combination of these connected by one or more operators. The expression cannot be a subquery.

I should also add that if this were to work as you would hope, it would present all kinds of new issues you’d have to deal with. Presently, updates across many columns/rows occur in parallel and atomically.

Therefore, it wouldn’t make sense to use a computed column in your calculation because it wouldn’t exactly have a value…yet. If anything, you’d be using an old, un-updated value.

If you really wanted to avoid duplicating the expression, you could do this in a trigger, though I strongly urge you not do that. Triggers are no fun and should only be used by very savvy people in rare cases.