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"
completion dates of tests = T1.date_of_completion; tests that have result 'Fail' refers to WHERE T2.test_result = 'Fail';