How to exclude rows containing null values within columns?
SELECT * FROM (
SELECT student_id,
       salutation,
       CASE
           WHEN salutation IN('Dr.') THEN 'Doctor'
       END AS NewSalutation
FROM student.student
) 
WHERE NewSalutation IS NOT NULL;


If that's your only section in the CASE statement, a logically equivalent query would be:

SELECT student_id,
       salutation,
       CASE
           WHEN salutation IN('Dr.') THEN 'Doctor'
       END AS NewSalutation
FROM student.student
WHERE salutation = 'Dr.';


... as anything other than Dr. will produce a NULL NewSalutation.