It is possible in SQL Server 2005 to create a computed column that is both persisted and is defined as NOT NULL (cannot contain a null value). The 2nd attribute is of importance when using libraries like Linq2Sql if we want to avoid a lot of manual work to assure our code that the column ‘always’ has a value.
With straight SQL this is very simple:
ALTER TABLE Sales ADD Total AS (Price + Taxes) PERSISTED NOT NULL
When viewed in the design window of SQL Server Management studio this column is correctly shown as a computed column with no checkmark for ‘allows nulls’. However I run into a problem creating new columns in the designer to match this pattern: The formula is entered in the Computed Column Specification -> (Formula) property, and the persisted property is specified by setting Is Persisted to Yes, but attempting to uncheck the ‘allows nulls’ on a new computed column results in a dialog stating “Property cannot be modified”.
I need to cover a wide range of skill levels, and to do so I need to provide procedures for adding columns that even a novice can follow (which means the Management Studio designer window). Is there some secret in SQL Server Management Studio for creating a new computed column as NOT NULL in the designer, similar to say how CTRL+0 can be used to insert nulls into cells?
You may cheat this with
ISNULL(Price + Taxes, 0)which uses the default value 0 for NULL computations.