Table schema:
CREATE TABLE Student_Tests_Taken (registration_id VARCHAR, test_result VARCHAR); CREATE TABLE Student_Course_Enrolment (date_of_completion VARCHAR, registration_id VARCHAR)

Question: What are the completion dates of all the tests that have result "Fail"?
SQL: 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"