MS SQL Server Concepts and Programming Question:

The rule for assigning NULL values to variables or table columns is simple: Use keyword "NULL" directly as normal values. Specifically,

* "NULL" can be used in SET statements to assign NULL values to variables.
* "NULL" can be used in SET clauses in UPDATE statements.
* "NULL" can be used in value lists in INSERT statements.
* "NULL" can be used in parameter lists when calling stored procedures or functions.

The tutorial script below gives you some good examples:

USE GlobalGuideLineDatabase;
GO

-- assign NULL values to variables
DECLARE @birth_date DATETIME;
SET @birth_date = NULL;
SELECT @birth_date;
GO
-----------------------
NULL

-- assign NULL values to columns
UPDATE ggl_links SET notes = NULL;
GO
(8 row(s) affected)

-- assign NULL values to parameters
EXEC sp_help NULL;
GO
Name
----------------
ggl_links_dump
ggl_links_top
ggl_links_view
...