CREATE TABLE Student_Course_Enrolment (date_of_completion VARCHAR, registration_id VARCHAR)
CREATE TABLE Student_Tests_Taken (registration_id VARCHAR, test_result VARCHAR)

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 "Fail" = "T2"."test_result" AND "T1"."registration_id" = "T2"."registration_id"