An IS NULL predicate tests for null values in value expressions.

  is-null-predicate =

  ----> value-expr ---> IS -+--------+-> NULL --->
                            +-> NOT -+

    See the Value_Expressions HELP topic for details on value
    expressions.

    SQL never evaluates an IS NULL predicate as unknown; it is always
    true or false. If the value expression is null, SQL evaluates
    the predicate as true. If the value expression is not null, the
    predicate is false.

    Use an IS NULL predicate to retrieve rows with null values in
    particular columns. An IS NULL predicate is the only way to
    construct a query that includes rows in a result table by testing
    whether or not particular columns in the rows have null values.
    Other constructions such as NOT LIKE or <> (not equal) do not
    include rows with null values in their result tables.