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@@ -0,0 +1,250 @@
+# 1、笔记
+
+```sql
+-- 计算平均值
+avg(x)
+	select avg(字段名) from 表名;
+-- 计算和
+sum(x)
+	#简单求和
+	select sum(字段名) from 表名;
+	#计算表达式求和
+	select sum(字段名*(1+ifnull(字段名,代替值)));
+-- 计算最大值
+max(x)
+	select max(字段名) from 表名;
+-- 计算最小值
+min(x)
+	select min(字段名) from 表名;
+-- 统计记录
+count(x)
+	#统计实际的行数
+	select count(*) from 表名;
+	#只统计"字段/表达式"部分非空值得行数
+	select count(字段/表达式) from 表名;
+	
+-- 内联查询
+select 字段名 from 表1 [inner] join 表2 on 条件; -- 显式内连接
+select 字段名 from 表1,表2 where 条件; -- 隐式内连接
+select 字段名 from 表1 natural join 表2; -- 自然连接
+-- 左外联查询
+select 字段名 from 表1 left join 表2 on 条件;
+-- 右外联查询
+select 字段名 from 表1 right join 表2 on 条件;
+-- 多表查询
+select 字段名 from 表1,表2,表3... where 表1.关联字段 = 表2.关联字段 and 表1.关联字段 = 表3.关联字段 ...;
+select 字段名 from 表1 inner join 表2 on 条件 
+					inner join 表3 on 条件...; 
+```
+
+
+
+# 2、作业
+
+```sql
+use mockq;
+--  1.设有一数据库，包括四个表：学生表（Student）、课程表（Course）、成绩表（Score）以及教师信息表（Teacher）。四个表的结构分别如表1-1的表（一）~表（四）所示，数据如表1-2的表（一）~表（四）所示。用SQL语句创建四个表并完成相关题目。
+-- 2.数据库的表结构
+-- 表（一）Student (学生表)
+-- 属性名	数据类型	可否为空	含义
+-- Sno	varchar (20)	否	学号（主码）
+-- Sname	varchar (20)	否	学生姓名
+-- Ssex	varchar (20)	否	学生性别
+-- Sbirthday	datetime	可	学生出生年月
+-- Class	varchar (20)	可	学生所在班级
+create table student(
+	Sno varchar(20) not null,
+	Sname varchar(20) not null,
+	Ssex varchar(20) not null,
+	Sbirthday datetime,
+	Class varchar(20),
+	primary key(Sno)
+);
+
+-- 表（二）Course（课程表）
+-- 属性名	数据类型	可否为空	含义
+-- Cno	varchar (20)	否	课程号（主码）
+-- Cname	varchar (20)	否	课程名称
+-- Tno	varchar (20)	否	教工编号（外码）
+-- drop table score;
+create table course(
+	Cno varchar(20) not null,
+	Cname varchar(20) not null,
+	Tno varchar(20) not null,
+	primary key(Cno),
+	foreign key(Tno) references teacher(Tno) on update cascade
+);
+-- alter table course add constraint fk_course_Tno foreign key(Tno) references teacher(Tno);
+-- alter table course drop foreign key fk_course_Tno;
+-- 表（三）Score(成绩表)
+-- 属性名	数据类型	可否为空	含义
+-- Sno	varchar (20)	否	学号（外码）
+-- Cno	varchar (20)	否	课程号（外码）
+-- Degree	Decimal(4,1)	可	成绩
+-- 主码：Sno+ Cno
+create table score(
+	Sno varchar(20) not null,
+	Cno varchar(20) not null,
+	Degree decimal(4,1),
+	foreign key(Sno) references student(Sno) on update cascade,
+	foreign key(Cno) references course(Cno) on update cascade
+);
+alter table score add primary key(Sno,Cno);
+-- 表（四）Teacher(教师表)
+-- 属性名	数据类型	可否为空	含义
+-- Tno	varchar (20)	否	教工编号（主码）
+-- Tname	varchar (20)	否	教工姓名
+-- Tsex	varchar (20)	否	教工性别
+-- Tbirthday	datetime	可	教工出生年月
+-- Prof	varchar (20)	可	职称
+-- Depart	varchar (20)	否	教工所在部门
+create table teacher(
+	Tno varchar(20) not null,
+	Tname varchar(20) not null,
+	Tsex varchar(20) not null,
+	Tbirthday datetime,
+	Prof varchar(20),
+	Depart varchar(20) not null,
+	primary key(Tno)
+);
+-- 3.数据库中的数据:
+-- -- 1,查询所有学生，都学了哪些课程，要显示学生信息和课程信息/
+select * from student inner join course;
+-- -- 2，查询没有学生的教师的所有信息
+select * from student,score,course,teacher where student.Sno = score.Sno and course.Cno = score.Cno and course.Tno = teacher.Tno;
+
+select * from student inner join score on student.Sno = score.Sno 
+											inner join course on course.Cno = score.Cno 
+											inner join teacher on course.Tno = teacher.Tno;
+
+-- 
+-- 表（一）Student
+-- Sno	Sname	Ssex	Sbirthday	class
+-- 108	曾华	男	1977-9-1	95033
+-- 105	匡明	男	1975-10-2	95031
+-- 107	王丽	女	1976-1-23	95033
+-- 101	李军	男	1976-2-20	95033
+-- 109	王芳	女	1975-2-10	95031
+-- 103	陆君	男	1974-6-3	95031
+insert into student values
+	('108','曾华','男','1977-9-1','95033'),
+	('105','匡明','男','1975-10-2','95031'),
+	('107','王丽','女','1976-1-23','95033'),
+	('101','李军','男','1976-2-20','95033'),
+	('109','王芳','女','1975-2-10','95031'),
+	('103','陆君','男','1974-6-3','95031');
+-- 表（二）Course
+-- Cno	Cname	Tno
+-- 3-105	计算机导论	825
+-- 3-245	操作系统	804
+-- 6-166	数字电路	856
+-- 9-888	高等数学	831
+insert into course VALUES
+	('3-105','计算机导论','825'),
+	('3-245','操作系统','804'),
+	('6-166','数字电路','856'),
+	('9-888','高等数学','831');
+-- 表（三）Score
+-- Sno	Cno	Degree
+-- 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
+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','64'),
+	('108','3-105','78'),
+	('101','6-166','85'),
+	('107','6-166','79'),
+	('108','6-166','81');
+-- 表（四）Teacher
+-- Tno	Tname	Tsex	Tbirthday	Prof	Depart
+-- 804	李诚	男	1958-12-2	副教授	计算机系
+-- 856	张旭	男	1969-3-12	讲师	电子工程系
+-- 825	王萍	女	1972-5-5	助教	计算机系
+-- 831	刘冰	女	1977-8-14	助教	电子工程系
+insert into teacher values
+	('804','李诚','男','1958-12-2','副教授','计算机系'),
+	('856','张旭','男','1969-3-12','讲师','电子工程系'),
+	('825','王萍','女','1972-5-5','助教','计算机系'),
+	('831','刘冰','女','1977-8-14','助教','电子工程系');
+-- 4.查询
+-- 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 student.Sname,score.Cno,score.Degree from student,score where score.Sno = student.Sno; -- 隐式内连接
+
+select student.Sname,score.Cno,score.Degree from student inner join score on score.Sno = student.Sno; -- 显式内连接
+
+select student.Sname,score.Cno,score.Degree from student natural join score; -- 自然连接
+-- ③　查询所有学生的Sno、Cname和Degree列。
+select score.Sno,course.Cname,score.Degree from score inner join course on score.Cno = course.Cno;
+-- ④　查询所有学生的Sname、Cname和Degree列。
+select student.Sname,course.Cname, score.Degree from student,course,score where student.Sno = score.Sno and course.Cno = score.Cno;
+-- ⑤　查询“95033”班学生的平均分。
+select avg(score.Degree) from score inner join student on student.Sno = score.Sno and student.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
+																inner join teacher on course.Tno = teacher.Tno
+																and teacher.Tname = '张旭';
+-- 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);
+-- 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;
+-- ⑮　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.
+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;
+-- ⑯　查询成绩比该课程平均成绩低的同学的成绩表。
+select * from score where Degree < (select avg(Degree) from score);
+-- ⑰　查询所有任课教师的Tname和Depart.
+select Tname,Depart from teacher;
+-- ⑱　查询所有未讲课的教师的Tname和Depart.
+select Tname,Depart from teacher inner join course on teacher.Tno = course.Tno and course.Cno is null;
+-- ⑲　查询“男”教师及其所上的课程。
+select teacher.Tname,course.Cname from course inner join teacher on teacher.Tno = course.Tno and teacher.Tsex = '男';
+-- ⑳　查询最高分同学的Sno、Cno和Degree列。
+select * from score where Degree = (select max(Degree) from score);
+-- 21　查询和“李军”同性别的所有同学的Sname.
+select Sname from student where Ssex = '男' and Sname != '李军';
+-- 22　查询和“李军”同性别并同班的同学Sname.
+select Sname from student where Ssex = '男' and Sname != '李军' and Class = '95033';
+-- 23　查询所有选修“计算机导论”课程的“男”同学的成绩表。
+select score.Cno,score.Sno,score.Degree from score inner join student on score.Sno = student.Sno
+										inner join course on score.Cno = course.Cno
+										and course.Cname = '计算机导论'
+										and student.Ssex = '男';
+```
+