数据准备：
先创建student和score表，添加基本数据，用于查询演练（参考答案如下）
创建student表SQL代码如下：
create table student(
id int(10) not null unique primary key,
name varchar(20) not null,
gender varchar(4),
birth year,
department varchar(20),
address varchar(50)
);
创建score表SQL代码如下：
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)
);
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'),
( 902,'张老二', '男',1986,'中文系', '北京市昌平区'),
( 903,'张三', '女',1990,'中文系', '湖南省永州市'),
( 904,'李四', '男',1990,'英语系', '辽宁省阜新市'),
( 905,'王五', '女',1991,'英语系', '福建省厦门市'),
( 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),
(NULL,901, '英语', 80),
(NULL,902, '计算机',65),
(NULL,902, '中文',88),
(NULL,903, '中文',95),
(NULL,904, '计算机',70),
(NULL,904, '英语',92),
(NULL,905, '英语',94),
(NULL,906, '计算机',90),
(NULL,906, '英语',85);
关卡一
1、查询student表的所有记录
selec * from student;
2、查询student表的第2条到第4条记录
select * from student limit 1,3;
3、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;
4、从student表中查询计算机系和英语系的学生的信息
select * from student where department in ('计算机系','英语系');
5、从student表中查询年龄22~26岁的学生信息
select
id,
name,
gender,
year(now())-birth as age
department,
address
from student
where year(now())-birth >= 10 and year(now())-birth <= 40;
关卡二：继续用关卡一的数据表进行查询演练
1、从student表中查询每个院系有多少人
select department,count(1) from student group by department;
2、从score表中查询每个科目的最高分
select c_name,max(grade) from score group by c_name;
3、查询李四的考试科目（c_name）和考试成绩(grade)--什么叫行级和列级子查询？
select c_name,grade from score where score.stu_id= (select id from student where name='李四');
4、用连接的方式查询所有学生的信息和考试信息
select s.*,c.* from student as s left join score as c on s.id=c.stu_id;
5、查询每个学生的总成绩
select s.name,sum(c.grade) from student as s left join score as c on s.id = c.stu_id group by s.name;
6、查询每个考试科目的平均成绩
select c_name,round(avg(grade),1) from score group by c_name;
7、查询计算机成绩低于95的学生信息-子查询
select s.* from student as s inner join score as c on s.id = c.stu_id where c.c_name='计算机' and grade<95;
select * from student where id in (select stu_id from score where c_name='计算机' and grade<95);
关卡三：
1、查询同时参加计算机和英语考试的学生的信息--一个表可以多次连接，如果需要筛选的条件为多个
select
stu.*
from student as stu
inner join score as s1
inner join score as s2
on stu.id = s1.stu_id
and stu.id = s2.stu_id
and s1.stu_id = s2.stu_id
where s1.c_name='计算机'
and s2.c_name='英语';
2、查询计算机考试成绩按从高到低进行排序
select stu_id,grade from score where c_name = '计算机' order by grade desc;
3、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 s.name,s.department,c.c_name,c.grade from student as s left join score as c on s.id = c.stu_id where name like '张%' or name like '王%';
4、查询都是湖南的学生的姓名、年龄、院系和科目及成绩--计算的就不用在前面加上表的简称
select s.name,year(now())-birth as age,s.department,c.c_name,c.grade from student as s left join score as c on s.id = c.stu_id where address like '湖南%';