经典Mysql语句(一)

表名和字段

–1.学生表
Student(s_id,s_name,s_birth,s_sex) --学生编号,学生姓名, 出生年月,学生性别
–2.课程表
Course(c_id,c_name,t_id) – --课程编号, 课程名称, 教师编号
–3.教师表
Teacher(t_id,t_name) --教师编号,教师姓名
–4.成绩表
Score(s_id,c_id,s_score) --学生编号,课程编号,分数

表数据

--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);

第一道sql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

一、

SELECT sa.`s_id`,c.`c_id`,sa.`s_score` AS 01_score ,c.`c_name` FROM course c  
INNER JOIN score sa ON c.`c_id` = sa.`c_id`

Course表内连接score表，挑出ON子句c.c_id = sa.c_id条件成立的记录，图中可以看出，查询出score表中c_id和course表中c_id相同的记录，也就是把每个人全部科目的成绩查询出来了

二、

SELECT sa.`s_id`,c.`c_id`,sa.`s_score` AS 01_score  ,c.`c_name` FROM course c  
INNER JOIN score sa ON c.`c_id` = sa.`c_id`  AND sa.`c_id`=01

添加 AND sa.c_id=01这个条件，就把score表中每个人的语文课程成绩查询出来

三、

SELECT sta.`s_name`,sa.`s_id`,c.`c_id`,sa.`s_score` AS 01_score  ,c.`c_name` FROM course c  
INNER JOIN score sa ON c.`c_id` = sa.`c_id`  
INNER JOIN student sta ON sta.`s_id` = sa.`s_id`

在刚才的基础上添加一个内连接student表，挑出ON子句sta.s_id = sa.s_id条件成立的记录，也就是每个学生全部科目的成绩，每个学生对应每一个科目，并且显示出学生名字，相当于score表通过s_id查出student表中学生的名字

四、

SELECT sta.`s_name`,sa.`s_id`,c.`c_id`,sa.`s_score` AS 01_score  ,c.`c_name` FROM course c  
INNER JOIN score sa ON c.`c_id` = sa.`c_id`  AND sa.`c_id`=01
INNER JOIN student sta ON sta.`s_id` = sa.`s_id`

添加 AND sa.c_id=01这个条件，就把score表中每个人的语文课程成绩查询出来

五、

SELECT sa.`s_id`,sta.`s_name`,c.`c_id`,sa.`s_score` AS 01_score , sb.`s_score` AS 02_score ,c.`c_name` FROM course c  
INNER JOIN score sa ON c.`c_id` = sa.`c_id`  AND sa.`c_id`=01
INNER JOIN student sta ON sta.`s_id` = sa.`s_id`
INNER JOIN score sb ON sta.`s_id`= sb.`s_id` AND sb.`c_id`=02

内连接另外一张scoure表，用来查询02数学成绩，此时挑出ON子句 sta.s_id= sb.s_id条件成立的记录，用student表和scoure表关联查询，能查询出对应每个s_id的学生数学成绩

六、

SELECT sa.`s_id`,sta.`s_name`,c.`c_id`,sa.`s_score` AS 01_score , sb.`s_score` AS 02_score ,c.`c_name` FROM course c  
INNER JOIN score sa ON c.`c_id` = sa.`c_id`  AND sa.`c_id`=01
INNER JOIN student sta ON sta.`s_id` = sa.`s_id`
INNER JOIN score sb ON sta.`s_id`= sb.`s_id` AND sb.`c_id`=02 
WHERE sa.`s_score` > sb.`s_score`

最后添加 WHERE sa.s_score > sb.s_score就能筛选出01成绩大于02成绩的学生信息

附录：

这是网上的答案

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

	select a.*,b.s_score as 01_score,c.s_score as 02_score from student a,score b,score c 
			where a.s_id=b.s_id 
			and a.s_id=c.s_id 
			and b.c_id='01' 
			and c.c_id='02' 
			and b.s_score>c.s_score

总结：

这个看似简单的题目，真正自己做起来就知道困难所在，本来只用到student表和scoure表，但是博主刚开始想的时候就把course表也加了进去，为了能显示课程名，所以本来两表的操作，最后被博主弄成了三表操作，其中内连接和左连接的用法也需要花点时间去捋一下。