SQL基础练习

--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);

1、查询"01"课程比"02"课程成绩高的学生的信息及课程分数：

SELECT C.*,D.Cname,D.TId from (SELECT Student.*,SC.CId,SC.score
FROM
Student
Left JOIN
SC
ON Student.SId=SC.SId
WHERE
Student.SId IN (SELECT A.SID from (SELECT * FROM SC WHERE CId ='01') A,(SELECT * FROM SC WHERE CId='02') B WHERE A.SID =B.SID AND A.SCORE>B.SCORE))
C LEFT JOIN Course D ON C.CId =D.CId;

思路：

1.先查出成绩表SC中01课程的成绩信息，再查出表中02课程的成绩信息

SELECT * FROM SC WHERE CId ='01'；

SELECT * FROM SC WHERE CId='02'；

2.通过内连接查出两张表sid相同的sid

SELECT A.SID from (SELECT * FROM SC WHERE CId ='01') A,(SELECT * FROM SC WHERE CId='02') B WHERE A.SID =B.SID AND A.SCORE>B.SCORE；

3.通过左链接链接Student，SC两表的信息

SELECT Student.*,SC.CId,SC.score 
FROM 
Student 
Left JOIN 
SC 
ON Student.SId=SC.SId 

4.查询符合条件的学生的信息和课程分数

SELECT Student.*,SC.CId,SC.score
FROM
Student
Left JOIN
SC
ON Student.SId=SC.SId
WHERE
Student.SId IN (SELECT A.SID from (SELECT * FROM SC WHERE CId ='01') A,(SELECT * FROM SC WHERE CId='02') B WHERE A.SID =B.SID AND A.SCORE>B.SCORE)

5.三表联查

SELECT C.*,D.Cname,D.TId from (SELECT Student.*,SC.CId,SC.score
FROM
Student
Left JOIN
SC
ON Student.SId=SC.SId
WHERE
Student.SId IN (SELECT A.SID from (SELECT * FROM SC WHERE CId ='01') A,(SELECT * FROM SC WHERE CId='02') B WHERE A.SID =B.SID AND A.SCORE>B.SCORE))
C LEFT JOIN Course D ON C.CId =D.CId;

查询结果：

2.查询平均成绩大于60分的同学的学号和平均成绩；

select sid,avg(score) from sc group by sid having avg(score)>60;

3.查询所有同学的学号、姓名、选课数、总成绩；

SELECT
A.SID,A.SNAME,B.ss,B.cc 
FROM 
(select sid,sname from Student) A
LEFT JOIN 
(select sid,SUM(score) ss,count(cid) cc from sc group by sid) B
WHERE A.SID =B.SID;