编号 姓名 性别 班级编号 1 name_1 male 1 2 name_2 female 2 3 name_3 male 3 4 name_4 female 4 5 name_5 male 1 6 name_6 female 2 7 name_7 male 3 8 name_8 female 4 9 name_9 male 1 10 name_10 female 2 11 name_11 male 3 12 name_12 female 4 13 name_13 male 1 14 name_14 female 2 15 name_15 male 3 16 name_16 female 4 17 name_17 male 1 18 name_18 female 2 19 name_19 male 3 20 name_20 female 4 21 name_21 male 1 22 name_22 female 2 23 name_23 male 3
班级编号 名称 1 服装 2 机械 3 计算机 5 航天
sql：select * from student where sex=‘male‘;
mr：该操作属于比较简单的操作，直接在map阶段对数据进行过滤操作即可。
sql：select class_name,count(*) from student group by class_name;
mr：
原文：http://www.cnblogs.com/lilei2blog/p/7804207.html