1.创建student

CREATE  TABLE  student (
id  INT(10)  NOT NULL  UNIQUE  PRIMARY KEY  ,
name  VARCHAR(20)  NOT NULL ,
sex  VARCHAR(4)  ,
birth  YEAR,
department  VARCHAR(20) ,
address  VARCHAR(50)
);

创建score表

CREATE  TABLE  score (
id  INT(10)  NOT NULL  UNIQUE  PRIMARY KEY  AUTO_INCREMENT ,
stu_id  INT(10)  NOT NULL ,
c_name  VARCHAR(20) ,
grade  INT(10)
);

2.为student表和score表增加记录

向student表插入记录的INSERT语句如下：

INSERT INTO student VALUES( 901,'张老大', '男',1985,'计算机系', '北京市海淀区');
INSERT INTO student VALUES( 902,'张老二', '男',1986,'中文系', '北京市昌平区');
INSERT INTO student VALUES( 903,'张三', '女',1990,'中文系', '湖南省永州市');
INSERT INTO student VALUES( 904,'李四', '男',1990,'英语系', '辽宁省阜新市');
INSERT INTO student VALUES( 905,'王五', '女',1991,'英语系', '福建省厦门市');
INSERT INTO student VALUES( 906,'王六', '男',1988,'计算机系', '湖南省衡阳市');

向score表插入记录的INSERT语句如下：

INSERT INTO score VALUES(NULL,901, '计算机',98);
INSERT INTO score VALUES(NULL,901, '英语', 80);
INSERT INTO score VALUES(NULL,902, '计算机',65);
INSERT INTO score VALUES(NULL,902, '中文',88);
INSERT INTO score VALUES(NULL,903, '中文',95);
INSERT INTO score VALUES(NULL,904, '计算机',70);
INSERT INTO score VALUES(NULL,904, '英语',92);
INSERT INTO score VALUES(NULL,905, '英语',94);
INSERT INTO score VALUES(NULL,906, '计算机',90);
INSERT INTO score VALUES(NULL,906, '英语',85);

-- 3.查询student表的所有记录

select * from student ;

-- 4.查询student表的第2条到4条记录

select * from student limit 1,3;

-- 5.从student表查询所有学生的学号（id）、姓名（name）和院系（department）的信息

select id, name, department from student;

-- 6.从student表中查询计算机系和英语系的学生的信息

select id ,name ,department from student where department in ('计算机系','英语系');

-- 7.从student表中查询年龄18~22岁的学生信息

select id,name,(2010-birth) '年龄' from student where 2010-birth between 18 and 22;

-- 8.从student表中查询每个院系有多少人

select department,count(*) '人数' from student group by department;

-- 9.从score表中查询每个科目的最高分

select max(grade),c_name from score group by c_name;

-- 10.查询李四的考试科目（c_name）和考试成绩（grade）

select name,c_name,grade from student t1  
	join score t2  on 
	t1.id = t2.stu_id where 
	name ='李四';
	
(select id ,name from student where name ='李四') t1 ;

select c_name,grade,t1.name from score ,
(select id ,name from student where name ='李四') t1
where stu_id = t1.id

-- 11.用连接的方式查询所有学生的信息和考试信息

SELECT NAME,
	c_name,
	grade 
FROM
	student
	INNER JOIN score ON student.id = score.stu_id 
LIMIT 0,3;

-- 12.计算每个学生的总成绩

select name ,sum(grade) from student 
	inner join score
	on student.id = score.stu_id
	group by name 

-- 13.计算每个考试科目的平均成绩

select c_name,avg(grade) from score 
	GROUP BY c_name

-- 14.查询计算机成绩低于95的学生信息

select t2.id,t2.name,t1.c_name,t1.grade from score t1 ,
(select name,id from student) t2
where t1.id = t2.id and t1.c_name='计算机' and grade <95;
-- 下面的写法更好,把筛选条件多的子集 当做新表来用,效率更高

select name,c_name,grade from student t2,
		(select stu_id, c_name,grade from score where c_name ='计算机' and grade <95) t1
where
	t2.id = t1.stu_id;
	
	# 内连接
select name,c_name,grade from student 
	join 	
score
	on student.id = score.stu_id
where score.c_name ='计算机' and score.grade <95

-- --> 重要 <-----------------------------------
-- 15.查询同时参加计算机和英语考试的学生的信息 

select * from student t 
	inner join 
(select stu_id from score where c_name ='计算机') t1 
	inner join 
(select stu_id from score where c_name='英语') t2 
	on t.id = t2.stu_id and t.id = t1.stu_id
	
# --- 记得使用 distinct 去重.-----
-- 16.将计算机考试成绩按从高到低进行排序

select distinct c_name,grade from score where c_name='计算机' order by grade desc;

-- 17.从student表和score表中查询出学生的学号，然后合并查询结果

select * from student t1 
	left join 
(select c_name,stu_id,grade from score) t2
  on t1.id = t2.stu_id

-- 18.查询姓张或者姓王的同学的姓名、院系和考试科目及成绩

select t1.name,t1.department,t2.c_name,t2.grade from score t2
	join 
(select id,name,department from student 
		where 
name like '张%' 
		or 
name like '王%') t1 
  on  t2.stu_id = t1.id 

-- 19.查询都是湖南的学生的姓名、年龄、院系和考试科目及成绩

	select t1.name,(2019-t1.birth) '年龄', t1.department,t2.c_name,t2.grade
	from score t2
		inner join 
	(select * from student where address like '湖南%') t1
	 on t2.stu_id = t1.id