一、多表关联查询
1.创建student和score表
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)
);
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
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');
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 * 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 * from student where year(current_date()) - year(birth) between 18 and 22;
查询结果为空：
8.从student表中查询每个院系有多少人
select department, count(*) as count from student group by department;
9.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;
10.查询李四的考试科目（c_name）和考试成绩（grade）
select c_name, grade from score where stu_id = (select id from student where name = '李四');
11.用连接的方式查询所有学生的信息和考试信息
select * from student join score on student.id = score.stu_id;
12.计算每个学生的总成绩
select student.id, student.name, sum(score.grade) from student join score on student.id = score.stu_id group by student.id;
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 * from student join score on student.id = score.stu_id where score.c_name = '计算机' and score.grade < 95;
15.查询同时参加计算机和英语考试的学生的信息
select * from student join score as s1 on student.id = s1.stu_id join score as s2 on student.id = s2.stu_id where s1.c_name = '计算机' and s2.c_name = '英语';
16.将计算机考试成绩按从高到低进行排序
select * 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 id from student union select stu_id from score;
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 student.name, student.department, score.c_name, score.grade from student join score on student.id = score.stu_id where student.name like '张%' or student.name like '王%';
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 student.name, year(current_date()) - student.birth as age, student.department, score.c_name, score.grade from student join score on student.id = score.stu_id where student.address like '%湖南%';
