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+# 1笔记
+
+聚合函数；多行，通过聚合会变成一行，或者少行
+
+1.求和 sum（） 
+
+```mysql
+select sum(salary) from temployee;
+```
+
+
+
+2.求平均值 avg（）
+
+```mysql
+select avg(salary) from employee;
+```
+
+
+
+3.求最大值 max（）
+
+```mysql
+select max(salary) from employee;
+```
+
+
+
+4.求最小值min（）
+
+```mysql
+select min(salary) from employee;
+```
+
+
+
+5。求数量 count（）
+
+```mysql
+select count(num) from employee;
+```
+
+保留两位小数
+
+```mysql
+select round(avg(salary),2) from empoyee;
+```
+
+
+
+6.分组；group by
+
+7 关联 ； inner join ...on	left join ...on 	right join ...on	 union
+
+```mysql
+select * from student inner join class on student.classid = classid;
+```
+
+
+
+# 2作业
+
+
+
+```mysql
+create database zhu charset utf8;
+show databases;
+use zhu;
+create table student(
+	sno varchar(20) primary key,
+	sname varchar(20) not null,
+	ssex varchar(20) not null,
+	sbirthday datetime null,
+	class varchar(20) null
+);
+insert into student values(108,'曾华','男','1977-9-1',95033);
+insert into student values(105,'匡明','男','1975-10-2',95031);
+insert into student values(107,'王丽','女','1976-1-23',95033);
+insert into student values(101,'李军','男','1976-2-20',95033);
+insert into student values(109,'王芳','女','1975-2-10',95031);
+insert into student values(103,'陆君','男','1974-6-3',95031);
+create table course(
+	cno varchar(20) not null primary key,
+	cname varchar(20) not null,
+	tno varchar(20) not null 
+	forign key (tno) references teacher(tno) 
+);
+insert into course values('3-105','计算机导论',825);
+insert into course values('3-245','操作系统',804);
+insert into course values('6-166','数字电路',856);
+insert into course values('9-888','高等数学',831);
+create table score(
+	sno varchar(20),
+	cno varchar(20),
+	degree decimal(4,1) null,
+	primary key(sno,cno)
+	foreign key (sno) references student(sno) on update cascade on delete cascade
+);
+
+insert into score values(103,'3-245',86),(105,'3-245',75),(109,'3-245',68),(103,'3-105',92),(105,'3-105',88),(109,'3-105',76),(101,'3-105',64),(107,'3-105',91),(108,'3-105',78),(101,'6-166',85),(107,'6-166',79),(108,'6-166',81);
+create table teacher(
+	tno varchar(20) primary key,
+	tname varchar(20) not null,
+	tsex varchar(20) not null,
+	tbirthday datetime null,
+	prof varchar(20) null,
+	depart varchar(20) not null
+);
+insert into teacher values(804,'李诚','男','1958-12-2','副教授','计算机系');
+insert into teacher values(856,'张旭','男','1969-3-12','讲师','电子工程系');
+insert into teacher values(825,'王萍','女','1972-5-5','助教','计算机系');
+insert into teacher values(831,'刘冰','女','1977-8-14','助教','电子工程系');
+#①　查询Score表中的最高分的学生学号和课程号。
+select sno,cno from score where  degree = (select max(degree) from score);
+#②　查询所有学生的Sname、Cno和Degree列。
+select sname,cno,degree from student inner join course on student.cno = course.cno;
+#③　查询所有学生的Sno、Cname和Degree列。
+select sno,cname,degree from student inner join score on student.sno = score.sno;
+#④　查询所有学生的Sname、Cname和Degree列。
+select sname,cname,degree from student inner join score on student.sno = score.sno and stuent.cno = course.cno;
+#⑤　查询“95033”班学生的平均分。
+select avg(degree) from student where class = 95033;
+#⑥　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。
+select * from student inner join score on student.Sno = score.Sno and score.Cno = '3-105' and score.Degree > 76;
+#⑦　查询score中选学多门课程的同学中分数为非最高分成绩的记录。
+select * from student inner join score on student.Sno = score.Sno and score.Degree = (select !(max(score.Degree)) from score);
+#⑧　查询成绩高于学号为“109”、课程号为“3-105”的成绩的所有记录。
+select * from student inner join score on student.Sno = score.Sno and score.Cno = '3-105' and score.Degree > 76;
+#⑨　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。
+select Sno,Sname,Sbirthday from student where year(Sbirthday) = '1977';
+#⑩　查询“张旭“教师任课的学生成绩。
+select score.Degree from score inner join course on score.Cno = course.Cno;
+																
+#11　查询选修某课程的同学人数多于5人的教师姓名。
+select teacher.Tname from teacher inner join course on teacher.Tno = course.Tno
+																	inner join score on course.Cno = score.Cno
+																	and score.Cno = (select count(Cno) > 5 from score);
+#12　查询出“计算机系“教师所教课程的成绩表。
+select score.Sno,score.Cno,score.Degree from course inner join score on course.Cno = score.Cno
+											inner join teacher on course.Tno = teacher.Tno and teacher.Depart = '计算机系';
+#13　查询“计算机系”与“电子工程系“不同职称的教师的Tname和Prof。
+select Tname,Prof from teacher where Prof in (select distinct Prof from teacher);
+#14　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。
+select score.Cno,score.Sno,score.Degree from score inner join course on score.Cno = course.Cno
+																																			and score.Cno = '3-105'
+																																			and score.Degree > 75 order by score.Degree desc;
+```
+