How to: Specify Computed Columns (Visual Database Tools)
You can specify an expression for a computed column in Table Designer. A computed column expression can use data from other columns to calculate a value for the column to which it belongs.
In Object Explorer, right-click the table with columns for which you want to specify a computed column and click Design (Modify in SP1 or earlier).
The table opens in Table Designer.
Select the column for which you want to specify a computed column formula.
In the Column Properties tab, expand the Computed Column Specification property.
In the (Formula) child property, type the expression for this column in the grid cell to the right. For example,
ColumnA + ColumnB.
Indicate whether the data is persisted by choosing Yes or No from the drop-down for the Is Persisted child property.
Note: If your Database Engine does not allow the persisting of expression results, you cannot choose Yes for the Is Persisted property.
The new computed column specification is assigned to the column in Table Designer after you click outside the grid cell or use the TAB key to move to another grid cell and changes are stored in the database when you save your changes in Table Designer.