CREATE TABLE students (student_id NUMBER, date_of_registration TIME, date_of_latest_logon TIME, login_name CLOB, password CLOB, personal_name CLOB, middle_name CLOB, family_name CLOB)
CREATE TABLE courses (course_id NUMBER, author_id NUMBER, subject_id NUMBER, course_name CLOB, course_description CLOB)
CREATE TABLE course_authors_and_tutors (author_id NUMBER, author_tutor_atb CLOB, login_name CLOB, password CLOB, personal_name CLOB, middle_name CLOB, family_name CLOB, gender_mf CLOB, address_line_1 CLOB)
CREATE TABLE student_course_enrolment (registration_id NUMBER, student_id NUMBER, course_id NUMBER, date_of_enrolment TIME, date_of_completion TIME)
CREATE TABLE student_tests_taken (registration_id NUMBER, date_test_taken TIME, test_result CLOB)
CREATE TABLE subjects (subject_id NUMBER, subject_name CLOB)

Return the completion date for all the tests that have 'Fail' result.
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" "T1" JOIN "student_tests_taken" "T2" ON "Fail" = "T2"."test_result" AND "T1"."registration_id" = "T2"."registration_id"