select count(教师姓名) from teacher;

select sum(成绩) from score;

select avg(成绩) from score;

select max(成绩),min(成绩) from score;

select count(distinct姓名) from student;

select sum(成绩) from score
where 课程号 = '0002';

/*这里用count计算有多少个学生号即可，但是一个学生可能会选择多个课程，
就会出现重复的学生号，所以要用到distinct 去除重复 */
select count(distinct 学号) as 学生人数 from score;

select 性别, count(*) as 学生人数 from student group by 性别;

select max(成绩) as 最高分, min(成绩) as 最低分 from score
group by 课程号;
-- 锁定成绩表之后，找到最高分最低分，再根据课程号分组

select 课程号,count(学生号) as 学生人数from score
group by 课程号;
-- 锁定成绩表之后，找到学号，再根据课程号分组

select 性别, count(*) as 学生人数 from student group by 性别;

select 性别, count(*) as 学生人数 from student
group by 性别 having count(*) > 1;

select 学号,avg(成绩) as 平均成绩 from score
group by 学号
having avg(成绩) > 60;
-- 从成绩标准找到学号以及成绩，将其按学号分类号后，应用函数 抽取大于60分的学生 

select 学号, count(课程号) as 选修课 from score
group by 学号
having count(课程号) >= 2;
-- 从成绩标准找到学号以及课程号，将其按学号分类号后，应用函数 抽取课程数量大于2的学生

select 姓名, count(*) as 人数 from student
group by 姓名
having count(*) >= 2;
/* 解题思路首先是 确定表student，然后根据题目所说的统计同名人数，那么就是按照姓名分组，
其次统计运用的就是count函数，同名的人数一定是两个及以上，
所以最后 用having提取计数两个以上的数据 */

select 课程,avg(成绩) from score
group by 课程号;

select 课程号,avg(成绩) from score
group by 课程号
having avg(成绩) >= 80;

select 课程号,avg(成绩) as 平均成绩 from score
group by 课程号
having avg(成绩) >= 80
order by 平均成绩 desc;;

select * from score
order by 成绩 asc, 课程号 desc;

select * from teacher order by 教师姓名;

select * from score
limit 2;

select 课程号 from score
where 成绩 < 60
order by 课程号 desc;

select 性别, count(*) as 学生人数 from student
group by 性别 having count(*) > 1;

select 性别, count(*) as 学生人数
from student
where count(*) > 1
group by 性别 ;