Allowing Null Values
The nullability of a column determines if the rows in the table can contain a null value for that column. A null value, or NULL, is not the same as zero (0), blank, or a zero-length character string such as ""; NULL means that no entry has been made. The presence NULL usually implies that the value is either unknown or undefined. For example, a null value in the price column of the titles table of the pubs database does not mean that the book has no price; NULL means that the price is unknown or has not been set. In general, avoid permitting null values because they incur more complexity in queries and updates and because there are other column options, such as PRIMARY KEY constraints, that cannot be used with nullable columns.
If a row is inserted but no value is included for a column that allows null values, Microsoft® SQL Server™ 2000 supplies the value NULL (unless a DEFAULT definition or object exists). A column defined with the keyword NULL also accepts an explicit entry of NULL from the user, no matter what data type it is or if it has a default associated with it. The value NULL should not be placed within quotation marks because it will be interpreted as the character string 'NULL', rather than the null value.
Specifying a column as not permitting null values can help maintain data integrity by ensuring that a column in a row always contains data. If null values are not allowed, the user entering data in the table must enter a value in the column or the table row cannot be accepted into the database.
Note Columns defined with a PRIMARY KEY constraint or IDENTITY property cannot allow null values.