A NULL value is a special value that represents an unknown value. SQL Server supports NULL values with the following features:

* All data types used for table columns support NULL values. In another word, NULL values can be stored in database tables.

* Individual table columns may be defined to not allow NULL values. In this case, you can not assign NULL values to those columns.

* "NULL" is a keyword that represent a NULL value in expressions.

* NULL values can be used directly in SET (assignment) statements.

* If NULL values are involved in an arithmetic operation, the result will be a numeric NULL.

* If NULL values are involved in a string operation, the result will be a string NULL.

* If NULL values are involved in a datetime operation, the result will be a datetime NULL.

* If NULL values are involved in a bitwise operation, the result will be a binary NULL.

* If NULL values are involved in a comparison operation, the result will be a Boolean NULL.

* If NULL values are involved in a Boolean operation, the result could be TRUE, FALSE, or NULL.

* To test NULL values, you need to use two special operators, IS NULL and IS NOT NULL.

* Special functions are available to handle NULL values, like ISNULL(), and NULLIF()