Order by sum

[Nano_]

Saludos

Prueba de la siguiente manera;

Código MySQL:

SELECT  t1.carrera
FROM tabla1 t1
LEFT JOIN (
SELECT tabla1.nick, sum( tabla2.creditos) as creditos
FROM tabla1
INNER JOIN tabla2 ON tabla1.nick= tabla2.autor 
GROUP BY tabla2.autor 
)t3 ON t1.nick= t3.nick
ORDER BY t3.creditos DESC