---恢复内容开始---
1,用于设定所select出来的数据是否允许出现重复行（完全相同的数据行）
all：允许出现——默认不写就是All（允许的）。
distinct：不允许出现——就是所谓的“消除重复行”
2，where:条件
3，group by：分组依据 后面加表的字段名，通常只进行一个字段的分组
mysql表查询语法形式：select [all | distinct] 字段名或表达式 from 表名 [where] [group by] [having] [order by] [limit];
练习题:共有下面四张表 学生表：student 教师表：teacher 课程表：course 成绩表：score
1，查询Score表中至少有5名学生选修的并以3开头的课程的平均分数
--操作表score，以cno分组并且cno是以3开头，取出各个cno的总数，及与cno对应的degree的平均值
select count(cno),avg(degree) from score where cno like '3%' group by cno;
--从上面的虚拟表中找到cno总数大于5的
select * from
(select count(cno) a,avg(degree) b from score where cno like '3%' group by cno) c
where a>5;
2,查询所有学生的Sno、Cname和Degree列
--找到student的sno
select sno from student;
--找到score 的sno,degree,cno
select sno,degree,cno from score;
--找到course的cno
select cno,cname from course;
--组成新表cno sno degree
select a.cno,b.sno,b.sname,a.degree from (select sno,degree,cno from score) a join (select sno,sname from student) b on a.sno=b.sno;
--组成有cname cno sn degree sname的表
select d.cname,e.cno,e.sno,e.degree,e.sname
from
(select a.cno,b.sno,b.sname,a.degree from (select sno,degree,cno from score) a join (select sno,sname from student) b on a.sno=b.sno) as e
join
(select cname,cno from course) as d
on d.cno=e.cno;
3，查询“95033”班学生的平均分
--从student取sno class,条件是class是95033的
select sno,class from student where class='95033';
--取出score中sno degree
select sno,degree from score;
--将上面两张表组成一张,取degree的平均值
select avg(degree) from
(select sno,class from student where class='95033') a
join
(select sno,degree from score) b
on a.sno=b.sno;
4,查询选修“3-105”课程的成绩高于“109”号同学成绩的所有同学的记录
--将109号的成绩取出
select degree from score where sno='109' and cno='3-105';
-- 得出最终表
select * from score
where
degree>(select degree from score where sno='109' and cno='3-105')
and cno='3-105';
5,查询和学号为108的同学同年出生的所有学生的Sno、Sname和Sbirthday列
--找到student中sno为108 的Sbirthday
select year(sbirthday) from student where sno='108';
-- 得出最终表
select sno,sname,sbirthday from student
where
year(sbirthday)=(select year(sbirthday) from student where sno='108');
6.查询“张旭“教师任课的学生成绩（姓名）
--找到teacher中tname为张旭的tno
select tno from teacher where tname='张旭';
--找到course中tno和teacher中tname为张旭的tno相同的cno
select cno from course where tno=(select tno from teacher where tname='张旭');
--找到score中cno为6-166的sno cno degree
select sno,cno,degree from score
where
cno=(select cno from course where tno=(select tno from teacher where tname='张旭'));
--找到student表中与上表sno相同的sname与上表组成新表
select a.sname,b.sno,b.cno,b.degree from
(select sno,sname from student) a
join
(select sno sno,cno,degree from score where cno=(select cno from course where tno=(select tno from teacher where tname='张旭'))) b
on a.sno=b.sno;
7,查询考计算机导论的学生成绩
--找到course中cname为计算机导论的cno
select cno from course where cname='计算机导论';
--找到score中cno与上表中相同时的sno degree
select sno,degree from score where cno=(select cno from course where cname='计算机导论');
-- 得出最终表
select b.sname,a.sno,a.degree from
(select sno,degree from score where
cno=(select cno from course where cname='计算机导论')) a join (select sname,sno from student) b
on a.sno=b.sno;
8,查询李诚老师教的课程名称
select tno from teacher where tname='李诚';
select cname,cno from course where tno=(select tno from teacher where tname='李诚');
9,查询选修某课程的同学人数多于5人的教师姓名
--score以cno分组，统计cno的数量
select count(cno) a,cno from score group by cno;
--找到上表中cno数量大于5的的cno
select cno from (select count(cno) a,cno from score group by cno) b where a>5;
--找到与上表cno一致的表course表中的tno
select tno from course where cno=(select cno from (select count(cno) a,cno from score group by cno) b where a>5);
--与上表tno一致的表teacher中的tname
select tname from teacher where tno=(select tno from course where cno=(select cno from (select count(cno) a,cno from score group by cno) b where a>5));
10,查询95033班和95031班全体学生的记录
--找到表score中的sno和cno 和degree
select sno,cno,degree from score;
--将上表和student组合
select a.sno,a.sname,a.ssex,a.sbirthday,a.class,b.cno,b.degree from
(select * from student) a join (select cno,degree,sno from score) b
on a.sno=b.sno;
11,查询存在有85分以上成绩的课程Cno
select cno,degree from score where degree>85;
12,查询出“计算机系“教师所教课程的成绩表
--找到teacher中的tno tname
select tno,tname from teacher where dapart='计算机系';
--找到course中tno和上表相同的cno
select a.cno,b.tno,b.tname from (select tno,cno from course) a join (select tno,tname from teacher where dapart='计算机系') b on a.tno=b.tno;
--找到score中cno与上表相同的degree
select c.sno,c.cno,c.degree,d.tname from (select sno,cno,degree from score) c join (select a.cno,b.tno,b.tname from (select tno,cno from course) a join (select tno,tname from teacher where dapart='计算机系') b on a.tno=b.tno) d on c.cno=d.cno;
13,查询选修编号为“3-105“课程且成绩至少高于选修编号为“3-245”的同学 的Cno、Sno和Degree,并按Degree从高到低次序排序
--score中找到cno=3-245的degree,取出最小值
select min(degree) from score where cno='3-245';
--score中找到cno=3-105的cno sno degree，并且degree大于上表的degree
select cno,sno,degree from
score where cno='3-105'
and degree>(select min(degree) from score where cno='3-245')
order by degree desc;
14,查询选修编号为“3-105”且成绩高于选修编号为“3-245”课程的同学的 Cno、Sno和Degree.
--找到score中cno=3-245中的degree的最大值
select max(degree) from score where cno='3-245';
--找到score中cno=3-105的cno sno degree
select cno,sno,degree from
score where
degree>(select max(degree) from score where cno='3-245') order by degree asc;
15,查询所有教师和同学的name、sex和birthday
select tname,tsex,tbirthday from teacher
union
select sname,ssex,sbirthday from student;
16,查询所有“女”教师和“女”同学的name、sex和birthday
select tname,tsex,tbirthday from teacher where tsex='女'
union
select sname,ssex,sbirthday from student where ssex='女';
17,查询成绩比该课程平均成绩低的同学的成绩表
select avg(degree) from score where cno='3-105';
select avg(degree) from score where cno='3-245';
select avg(degree) from score where cno='6-166';
select * from score where cno='3-105' and degree<(select avg(degree) from score where cno='3-105')
union
select * from score where cno='3-245' and degree<(select avg(degree) from score where cno='3-245')
union
select * from score where cno='6-166' and degree<(select avg(degree) from score where cno='6-166');
18,查询所有任课教师的Tname和Depart
select a.tname,a.dapart from
(select tno,tname,dapart from teacher) a join
(select tno from course) b
on a.tno=b.tno;
19,查询所有未讲课的教师的Tname和Depart
select a.tname,a.dapart from
(select tno,tname,dapart from teacher) a join
(select tno from course) b
on a.tno=b.tno;
20,查询至少有2名男生的班号
select count(ssex),class from student where ssex='男' group by class;
select b.class from (select count(ssex) a,class from student where ssex='男' group by class) b where a>=2;
21,查询Student表中不姓“王”的同学记录
select * from student where sname not like '王%';
22,查询Student表中每个学生的姓名和年龄
select sname,year(now())-year(sbirthday) from student;
23,查询Student表中最大和最小的Sbirthday日期值
select min(day(sbirthday)),max(day(sbirthday)) from student;
24,以班号和年龄从大到小的顺序查询Student表中的全部记录
select * from student order by class desc,date(sbirthday) asc;
25,查询“男”教师及其所上的课程
select tno from teacher where tsex='男';
select a.tno,b.cname,a.tname from (select tno,tname from teacher where tsex='男') a join (select * from course) b on a.tno=b.tno;
