---数据分组-统计信息-------select 字段列表from 表列表 where 对数据源进行数据筛选group by 分组字段列表Order by 排序字段列表--1.得到所有学员总人数select COUNT(*) from Student--2.得到男女生的人数select COUNT(*) from Student where Sex=‘男‘select COUNT(*) from Student where Sex=‘女‘--使用分组统计select COUNT(*),sex from Student group by sexselect COUNT(*),sex from Studentselect COUNT(*) from Studentselect distinct sex from Student--查询每个班级的人数--分组统计有一个规则:与聚合函数一起出现在查询中的列，要么被聚合，要么被分组--1.聚合不应出现在WHERE 子句中,语法规则--2.where 的执行在分组之前，先对源数据做筛选之后再对筛选得到的结果集做分组--3.having是对分组统计得到的结果集做筛选的。--select 字段列表from  表列表 where 源数据筛选条件 group by 分组统计字段列表having  对分组统计结果集做筛选order by 得到最终结果集之后的数据重排select classid ,COUNT(*)  from Student  group by ClassId  having ClassId=6select classid 班级,COUNT(*) 人数,StudentNo from Student  group by ClassId  having StudentNo>10--  5 显示                                       1获取数据源   2  筛选原                   3 对数据源进行分组 4 对分组统计的结果集做筛选   6对最终结果集做数据重排select classid 班级,COUNT(*) 人数 from Student where Email is not null  group by ClassId  having COUNT(*) between 2 and 3      order by  人数 descselect StudentNo no,StudentName name from Student where StudentName like ‘%‘select COUNT(*) 人数 from Student where Email is not null  having COUNT(*)>=3       order by  人数 descselect classid, sex,COUNT(*) from Student group by ClassId,Sex order by ClassId,sex--分组统计练习:--1.查询每个班级的总学时数，并按照升序排列select classid, SUM(ClassHour) from Subject where ClassId is not null group by ClassId order by SUM(ClassHour)--2.查询每个参加考试的学员的平均分select studentNo, AVG(StudentResult) from Result where StudentResult is not null group by StudentNo --having StudentResult is not null--3.查询每门课程的平均分，并按照降序排列select SubjectId, AVG(StudentResult) as score from Result where StudentResult is not null group by SubjectId order by  score desc--4.查询每个班级男女生的人数select ClassId,Sex, COUNT(*) from Student group by ClassId,Sex