问带分组的MySQL排名

EN

SELECT Branch, Grade, Section, RollNo, FIrst_Name, Father_Name, Grand_Father_Name, AverageMark, rank 
FROM (
    SELECT Branch, Grade, Section, RollNo, FIrst_Name, Father_Name, Grand_Father_Name, AverageMark, @curRank := IF(@prevRank = AverageMark, @curRank, @incRank) AS rank,  @incRank := @incRank + 1,  @prevRank := AverageMark 
    FROM studentsaverage p, 
        ( SELECT @curRank :=0, @prevRank := NULL, @incRank := 1 ) r 
    ORDER BY AverageMark DESC
) s