9. Write a trigger that prohibits duplicate values except for nulls in the NoDupName column of the following table: CREATE TABLE TestUniqueNulls (RowID int IDENTITY NOT NULL, NoDupName varchar(20) NULL); (Note that you can't do this by using a unique constraint because the constraint wouldn't allow duplicate null values.) If an INSERT or UPDATE statement creates a duplicate value, roll back the statement and return an error message. Write a series of INSERT statements that tests that duplicate null values are allowed but duplicates of other values are not.