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select  sClass 班级,count(*)  班级学生总人数,
sum(case when sGender=0 then 1 else 0 end) 女生人数,
sum(case when sGender=0 then 1 else 0 end)*1.0/count(*)女生所占比例,
sum(case when sGender=1 then 1 else 0 end) 男生人数,
sum(case when sGender=1 then 1 else 0 end)*1.0 /count(*)男生所占比例  
from student GROUP BY sClass ORDER BY sClass asc

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