How many tests have result "Fail"?
select count(*) from student_tests_taken where test_result = "Fail"
What are the dates of those tests?
select date_test_taken from student_tests_taken where test_result = "Fail"
What are the completion dates of all the tests that have result "Fail"?
select t1.date_of_completion from student_course_enrolment as t1 join student_tests_taken as t2 on t1.registration_id = t2.registration_id where t2.test_result = "Fail"