mysql count函数同字段不同值计数
问题：
某个字段分组多种级别，要统计不同级别总数
解答如下：
使用sum if 等关键函数进行操作
select exam_id,subject_name,sum(if(grade = 1,1,0)) as '1级',sum(if(grade = 2,1,0)) as '2级',sum(if(grade = 3,1,0)) as '3级',sum(if(grade = 4,1,0)) as '4级',sum(if(grade = 5,1,0)) as '5级'
,sum(if(grade = 6,1,0)) as '6级',sum(if(grade = 7,1,0)) as '7级',sum(if(grade = 8,1,0)) as '8级',sum(if(grade = 9,1,0)) as '9级',sum(if(grade = 10,1,0)) as '10级'
from biz_exam_student
group by exam_id,subject_id order by exam_id ,subject_id;
缺点：
效率不好，还有待优化；
最后
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