Developers can specify whether to allow or disallow null values, for each column in their table. A null value, or NULL, is not the same as zero (0) or blank; NULL means that no entry has been made. Its presence usually implies that the value is either unknown or undefined. If null values are not allowed, the user entering data in the table must enter a value in the field or the record cannot be accepted in the database. Developers cannot change this property on a primary key column. Also, identity columns cannot have null values. When developers working with SQL Server “Denali”, MS SQL Server 2008 R2 and MS SQL Server 2005 want modify the null options on a column, they should follow the next steps:
1. In Object Explorer, right-click the table with columns for which they want to modify the length and click Design (Modify in SP1 or earlier).
The table opens in Table Designer.
2. Select the column and in the Column Properties tab either clear or check the Allow nulls check box.
The new value for the Allow nulls check box is assigned to the column in the Table Designer after developers click outside the grid cell or use the TAB key to move to another grid cell. It takes effect in the database when they save their changes in the Table Designer.