Which major has the highest number of students?
select major from student group by major order by count ( * ) desc limit 1
What is the average age of the students in this major?
select avg ( age ) from student where major = ( select major from student group by major order by count ( * ) desc limit 1 )
What is the sex of the students in this major?
select sex from student where major = ( select major from student group by major order by count ( * ) desc limit 1 )