Specifies that NULL is permitted for the column. This is the
    default behavior. A column with a NULL constraint cannot also
    have a NOT NULL constraint within the same ALTER TABLE statement.
    However, no checks are performed for CHECK constraints, which may
    limit the column to non-null values.

    The NULL constraint is not stored in the database and is provided
    only as a syntactic alternative to NOT NULL.

    When used on ALTER TABLE . . . ALTER COLUMN this clause drops any
    NOT NULL constraints defined for the column.