packet ordering function plsql oracle

like this

/* 

id                   年段   班级

-------------------- ---- ----

1                    高一   1班

2                    高一   2班

1                    高二   1班

2                    高二   2班

1                    高三   1班

2                    高三   2班

 

 

SELECT grade,
class,
DENSE_RANK () the OVER (Grade ORDER BY BY Partition class) AS No. Class

table