CREATE TABLE student_course_registrations (student_id VARCHAR, course_id VARCHAR, registration_date VARCHAR)
CREATE TABLE courses (course_id VARCHAR, course_name VARCHAR)

List the id of students who registered course statistics in the order of registration date.
SELECT T2.student_id FROM courses AS T1 JOIN student_course_registrations AS T2 ON T1.course_id = T2.course_id WHERE T1.course_name = "statistics" ORDER BY T2.registration_date
SELECT "T2"."student_id" FROM "courses" AS "T1" JOIN "student_course_registrations" AS "T2" ON "T1"."course_id" = "T2"."course_id" WHERE "T1"."course_name" = "statistics" ORDER BY "T2"."registration_date" NULLS FIRST