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"
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"
[{'date_of_completion': '2018-03-02 14:38:49'}, {'date_of_completion': '2018-03-22 18:04:54'}, {'date_of_completion': '2018-03-22 18:04:54'}, {'date_of_completion': '2018-02-26 23:46:25'}, {'date_of_completion': '2018-03-07 09:45:48'}, {'date_of_completion': '2018-03-06 00:27:09'}, {'date_of_completion': '2018-03-04 01:24:56'}]