solución reto:
SELECT AVG(alumnos_por_tutor) AS promedio_alumnos
FROM(
SELECT CONCAT(t.nombre,' ', t.apellido) AS tutor,
COUNT(*) AS alumnos_por_tutor
FROM platzi.alumnos AS a
INNER JOIN platzi.alumnos AS t ON a.tutor_id=t.id
GROUP BY tutor
ORDER BY alumnos_por_tutor DESC
) AS promedio
--Experimento--
--promedio de tutores por carrera--
SELECT AVG(dc.tutor_por_carrera)
FROM(
-- seleciona cuantas tutorias realiza en una carrera--
SELECT CONCAT(t.nombre,' ', t.apellido) AS tutor,
COUNT(*) AS tutor_por_carrera,
a.carrera_id
FROM platzi.alumnos AS a
INNER JOIN platzi.alumnos AS t ON a.tutor_id=t.id
AND a.carrera_id=t.tutor_id
GROUP BY tutor, a.carrera_id
ORDER BY tutor_por_carrera DESC) AS dc
