use Myschool --------------------上机练习1------------------- --查询每个年级的总学时数,并按照升序排列 select gradeid as 年级,sum(Classhour) from subject group by gradeid order by SUM(Classhour) --查询每个参加考试的学员的平均分 select studentno as 学号,AVG(studentresult) from result group by studentno select * from subject --查询每门课程的平均分,并按照将序排列 select subjectid as 课程,AVG(studentresult) from result group by subjectid order by AVG(studentresult) desc --查询每个学生参加的所有考试的总分,并按照降序排列 select studentno as 学号,SUM(studentresult) from result group by studentno order by SUM(studentresult) desc ---------上机练习2------------------ --查询每学期学时超过50的课程数 use myschool select gradeid as 年级,COUNT(subjectid) as 课程数 from subject where classhour>50 group by gradeid --查询课程表的所有信息 select * from subject --查询每学期学生的平均年龄 select * from student select gradeid as 年级,AVG(DATEDIFF(yy,birthday,getdate()))as 平均年龄 from student group by gradeid --查询北京地区的每学期学生人数 select gradeid as 年级,COUNT(1) as 人数 from student where address like('%北京%') group by gradeid --查询参加考试的学生中,平均分及格的学生记录,并按照成绩降序排列 select studentno,AVG(StudentResult) as 平均分 from Result group by StudentNo having AVG(StudentResult)>=60 order by 平均分 desc --查询成绩表中的所有信息 select * from result --查询开始日期为2014年2月22日的课程的及格平均分 select subjectid,AVG(studentresult) as 平均分 from Result where ExamDate>='2014-2-22' and ExamDate<'2014-2-23' group by SubjectId having AVG(StudentResult)>=60 --统计至少有一次不及格的学生学号和次数。 select studentno,COUNT(1) as 次数 from Result where StudentResult<60 group by StudentNo
注意: (1)where之后不能跟聚合函数
(2) having是对分组后的数据进行第二次筛选或者过滤,也就是说没有group by就没having
(3)如果语句中有group by关键字,那么select后只能跟group by后出现的列,或者是聚合函数
SQL语句的书写顺序: 执行顺序:
select 列名或聚合函数 (4)投影结果
from 表名 (1)定位到表
where 条件 (2)分组前的第一道过滤
group by 列名 (3)分组
having 聚合函数或者分组后的列名(5)分组后的第二道过滤
order by (6)最后排序