IS [NOT] NULL (Transact-SQL)

expression

Is any valid expression.

NOT

Specifies that the Boolean result be negated. The predicate reverses its return values, returning TRUE if the value is not NULL, and FALSE if the value is NULL.

Boolean

If the value of expression is NULL, IS NULL returns TRUE; otherwise, it returns FALSE.

If the value of expression is NULL, IS NOT NULL returns FALSE; otherwise, it returns TRUE.

To determine whether an expression is NULL, use IS NULL or IS NOT NULL instead of comparison operators (such as = or !=). Comparison operators return UNKNOWN when either or both arguments are NULL.

The following example returns the name and the weight for all products for which either the weight is less than 10 pounds or the color is unknown, or NULL.

USE AdventureWorks2012;
GO
SELECT Name, Weight, Color
FROM Production.Product
WHERE Weight < 10.00 OR Color IS NULL
ORDER BY Name;
GO

Reference