What are the student IDs of students who passed the test?
SELECT T1.student_id FROM Student_Course_Enrolment AS T1 JOIN Student_Tests_Taken AS T2 ON T1.registration_id   =   T2.registration_id WHERE T2.test_result   =   "Pass"
student IDs of students who passed the test refers to selecting student_id from a join between Student_Course_Enrolment and Student_Tests_Taken where the test_result is 'Pass';