This is a classic example of an exclusion list and point sout the limitiations of the inequality operator. What is presented in this example is that members in one are are to be excluded from from others. (Note that the example is students who have not taken the SQL Server exam. Bob gets omitted. Does he get omitted because he has not taken an exam?).  And as the author points out, "you can simply use a subquery to build the list of students who have taken SQL Server exam" and then build your exclusion using the NOT EXISTS clause:

SELECT s.*

FROM Students s

where

 not exists ( select stid from studentExam se where s.StID = stid and examname = 'SQL Server' )