SELECT
    alun.nome, (SELECT AVG(hist_aux.nota) FROM HISTORICO hist_aux WHERE alun.matricula = hist_aux.matricula) AS Media
FROM
    HISTORICO hist,
    ALUNO alun,
    TURMA turma
WHERE
    alun.matricula = hist.matricula AND
    hist.cod_turma = turma.cod_turma AND
    turma.ano = 2013
ORDER BY Media DESC;