mysql数据库中limit的问题

如图所示我如何取得前20%，40%，这是MYSQL数据库,另外附上我的sql代码
SELECT
sum(sns_examdetail.SCORE) as student_score,
B.exam_name as exam_name,
sns_examdetail.STUDENT_ID as student_id
FROM
sns_exam ,
(select count(A.student_id) as student_count,A.exam_name as exam_name from sns_examdetail,sns_exam,
(SELECT
sns_examdetail.EXAM_ID as exam_id,
sns_exam.EXAM_NAME as exam_name,
sns_examdetail.STUDENT_ID AS student_id
FROM
sns_exam ,
sns_examdetail
where
sns_exam.EXAM_ID = sns_examdetail.EXAM_ID
and
sns_examdetail.PROBLEM_ID is null
and
sns_examdetail.STATUS <> 0
and
sns_exam.EXAM_NAME = '海淀'
and
sns_exam.AREA_ID = '13'
group by student_id) as A
where 
A.exam_id = sns_exam.EXAM_ID
and
A.student_id = sns_examdetail.STUDENT_ID
group by exam_name) as B,
sns_examdetail
where
sns_exam.EXAM_ID = sns_examdetail.EXAM_ID
and
B.exam_name = sns_exam.EXAM_NAME
and
sns_examdetail.STATUS <> 0
and
sns_examdetail.PROBLEM_ID is null
and
sns_exam.EXAM_NAME = '海淀'
and
sns_exam.AREA_ID = '13'
group by student_id
order by student_score desc

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