普通 + 中等 难度练习题
测试数据：
--建表
--学生表
CREATE TABLE `Student`(
`s_id` VARCHAR(20),
`s_name` VARCHAR(20) NOT NULL DEFAULT '',
`s_birth` VARCHAR(20) NOT NULL DEFAULT '',
`s_sex` VARCHAR(10) NOT NULL DEFAULT '',
PRIMARY KEY(`s_id`)
);
--课程表
CREATE TABLE `Course`(
`c_id` VARCHAR(20),
`c_name` VARCHAR(20) NOT NULL DEFAULT '',
`t_id` VARCHAR(20) NOT NULL,
PRIMARY KEY(`c_id`)
);
--教师表
CREATE TABLE `Teacher`(
`t_id` VARCHAR(20),
`t_name` VARCHAR(20) NOT NULL DEFAULT '',
PRIMARY KEY(`t_id`)
);
--成绩表
CREATE TABLE `Score`(
`s_id` VARCHAR(20),
`c_id` VARCHAR(20),
`s_score` INT(3),
PRIMARY KEY(`s_id`,`c_id`)
);
--插入学生表测试数据
insert into Student values('01' , '赵雷' , '1990-01-01' , '男');
insert into Student values('02' , '钱电' , '1990-12-21' , '男');
insert into Student values('03' , '孙风' , '1990-05-20' , '男');
insert into Student values('04' , '李云' , '1990-08-06' , '男');
insert into Student values('05' , '周梅' , '1991-12-01' , '女');
insert into Student values('06' , '吴兰' , '1992-03-01' , '女');
insert into Student values('07' , '郑竹' , '1989-07-01' , '女');
insert into Student values('08' , '王菊' , '1990-01-20' , '女');
--课程表测试数据
insert into Course values('01' , '语文' , '02');
insert into Course values('02' , '数学' , '01');
insert into Course values('03' , '英语' , '03');
--教师表测试数据
insert into Teacher values('01' , '张三');
insert into Teacher values('02' , '李四');
insert into Teacher values('03' , '王五');
--成绩表测试数据
insert into Score values('01' , '01' , 80);
insert into Score values('01' , '02' , 90);
insert into Score values('01' , '03' , 99);
insert into Score values('02' , '01' , 70);
insert into Score values('02' , '02' , 60);
insert into Score values('02' , '03' , 80);
insert into Score values('03' , '01' , 80);
insert into Score values('03' , '02' , 80);
insert into Score values('03' , '03' , 80);
insert into Score values('04' , '01' , 50);
insert into Score values('04' , '02' , 30);
insert into Score values('04' , '03' , 20);
insert into Score values('05' , '01' , 76);
insert into Score values('05' , '02' , 87);
insert into Score values('06' , '01' , 31);
insert into Score values('06' , '03' , 34);
insert into Score values('07' , '02' , 89);
insert into Score values('07' , '03' , 98);
视图展示：
# 学生表
mysql> select * from student;
+------+--------+------------+-------+
| s_id | s_name | s_birth | s_sex |
+------+--------+------------+-------+
| 01 | 赵雷 | 1990-01-01 | 男 |
| 02 | 钱电 | 1990-12-21 | 男 |
| 03 | 孙风 | 1990-05-20 | 男 |
| 04 | 李云 | 1990-08-06 | 男 |
| 05 | 周梅 | 1991-12-01 | 女 |
| 06 | 吴兰 | 1992-03-01 | 女 |
| 07 | 郑竹 | 1989-07-01 | 女 |
| 08 | 王菊 | 1990-01-20 | 女 |
+------+--------+------------+-------+
8 rows in set (0.00 sec)
# 老师表
mysql> select * from teacher;
+------+--------+
| t_id | t_name |
+------+--------+
| 01 | 张三 |
| 02 | 李四 |
| 03 | 王五 |
+------+--------+
3 rows in set (0.00 sec)
# 课程表
mysql> select * from course;
+------+--------+------+
| c_id | c_name | t_id |
+------+--------+------+
| 01 | 语文 | 02 |
| 02 | 数学 | 01 |
| 03 | 英语 | 03 |
+------+--------+------+
3 rows in set (0.00 sec)
# 成绩表
mysql> select * from score;
+------+------+---------+
| s_id | c_id | s_score |
+------+------+---------+
| 01 | 01 | 80 |
| 01 | 02 | 90 |
| 01 | 03 | 99 |
| 02 | 01 | 70 |
| 02 | 02 | 60 |
| 02 | 03 | 80 |
| 03 | 01 | 80 |
| 03 | 02 | 80 |
| 03 | 03 | 80 |
| 04 | 01 | 50 |
| 04 | 02 | 30 |
| 04 | 03 | 20 |
| 05 | 01 | 76 |
| 05 | 02 | 87 |
| 06 | 01 | 31 |
| 06 | 03 | 34 |
| 07 | 02 | 89 |
| 07 | 03 | 98 |
+------+------+---------+
18 rows in set (0.00 sec)
练习题：
# 1. 查询“01”课程比“02”课程成绩高的学生的信息及课程分数
# a代表学生表（student） b代表成绩表（score）
select a.*,b.s_score as 01_score ,c.s_score as 02_score from
student a join score b on b.s_id = a.s_id and b.c_id='01' left join score c on a.s_id=c.s_id and c.c_id="02" or c.c_id = NULL where b.s_score>c.s_score;
# 答案：
select a.* ,b.s_score as 01_score,c.s_score as 02_score from
student a join score b on a.s_id=b.s_id and b.c_id='01'
left join score c on a.s_id=c.s_id and c.c_id='02' or c.c_id = NULL where b.s_score>c.s_score;
# 2. 查询"01"课程比"02"课程成绩低的学生的信息及课程分数
# a代表学生表（student） b代表成绩表（score） c代表成绩表（score）
select a.*,b.s_score as score01,c.s_score as score02 from
student a join score b on a.s_id = b.s_id and b.c_id = "01"
join score c on a.s_id = c.s_id and c.c_id="02" where c.s_score > b.s_score;
# 答案
select a.* ,b.s_score as 01_score,c.s_score as 02_score from
student a left join score b on a.s_id=b.s_id and b.c_id='01' or b.c_id=NULL
join score c on a.s_id=c.s_id and c.c_id='02' where b.s_score<c.s_score;
# 3. 查询平均成绩大于等于60分的同学的学生编号和学生姓名和平均成绩
# a代表学生表（student） b代表成绩表（score） c代表成绩表（score）
select a.s_id,a.s_name,AVG(b.s_score) as average from student a join score b on a.s_id = b.s_id group by b.s_id HAVING average>60;
# 答案
select b.s_id,b.s_name,ROUND(AVG(a.s_score),2) as avg_score from
student b
join score a on b.s_id = a.s_id
GROUP BY b.s_id,b.s_name HAVING avg_score >=60;
# 4. 查询平均成绩小于60分的同学的学生编号和学生姓名和平均成绩-- (包括有成绩的和无成绩的)
# a代表学生表（student） b代表成绩表 (score)
select a.s_id,a.s_name,AVG(b.s_score) as average from student a left join score b on a.s_id = b.s_id group by b.s_id HAVING average < 60
# 答案：
select b.s_id,b.s_name,ROUND(AVG(a.s_score),2) as avg_score from
student b
left join score a on b.s_id = a.s_id
GROUP BY b.s_id,b.s_name HAVING avg_score <60
union
select a.s_id,a.s_name,0 as avg_score from
student a
where a.s_id not in (
select distinct s_id from score);
# 5.查询所有同学的学生编号、学生姓名、选课总数、所有课程的总成绩
# a代表学生表（student） b代表课程表 (score)
select a.s_id,a.s_name,count(b.c_id) as sum_course,SUM(b.s_score) as sum_score from student a left join score b on a.s_id = b.s_id group by a.s_id,a.s_name;
# 答案
select a.s_id,a.s_name,count(b.c_id) as sum_course,sum(b.s_score) as sum_score from
student a
left join score b on a.s_id=b.s_id
GROUP BY a.s_id,a.s_name;
# 6.查询"李"姓老师的数量
select count(*) from teacher where t_name like "李%";
# 答案：
select count(t_id) from teacher where t_name like '李%';