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

List each test result and its count in descending order of count.
SELECT test_result, COUNT(*) FROM student_tests_taken GROUP BY test_result ORDER BY COUNT(*) DESC
SELECT "test_result", COUNT(*) FROM "student_tests_taken" GROUP BY "test_result" ORDER BY COUNT(*) DESC NULLS LAST