子查询是一个嵌套在SELECT，INSERT，UPDATE，或DELETE语句或其他子查询中的查询。

创建数据库，表：

create database two20210610 default charset=utf8;

use two20210610;
CREATE TABLE `studentinfo` (
  `StudentID` char(10) DEFAULT NULL,
  `StudentName` varchar(20) DEFAULT NULL,
  `Gender` varchar(2) DEFAULT NULL,
  `Birthday` date DEFAULT NULL,
  `ClassID` int(4) DEFAULT NULL,
  `BeginYear` year(4) DEFAULT NULL,
  `Phone` varchar(11) DEFAULT NULL,
  `Province` varchar(20) DEFAULT NULL,
  `City` varchar(20) DEFAULT NULL,
  `Email` varchar(50) DEFAULT NULL
) ENGINE=InnoDB DEFAULT CHARSET=utf8;

create table teacher (
    id int,
    teachername VARCHAR(10),
    hiredate DATE,
    gender char

);

create table classinfo (
    id int,   # 班级编号
    classname varchar(10),
    gradeid int,   # 年级编号
    beginyear varchar(10)  # 开始年份

);

create table subject (
    id int ,
    subjectname varchar(10),
    teacherid int 
    
);

create table exam (
    id int,
    exam int,
    subjectid int,
    studentid int,
    remark varchar(10) # 备注
);

create table grade(
    id int ,
    gradename varchar(10),
    major varchar(20)
);

添加数据：

INSERT INTO `two20210610`.`classinfo`(`id`, `classname`, `gradeid`, `beginyear`) VALUES (2001, '20级1班', 20, '2020');
INSERT INTO `two20210610`.`classinfo`(`id`, `classname`, `gradeid`, `beginyear`) VALUES (2002, '20级2班', 20, '2020');

INSERT INTO `two20210610`.`exam`(`id`, `exam`, `subjectid`, `studentid`, `remark`) VALUES (1, 98, 101, 1, '无');
INSERT INTO `two20210610`.`exam`(`id`, `exam`, `subjectid`, `studentid`, `remark`) VALUES (2, 89, 102, 1, NULL);
INSERT INTO `two20210610`.`exam`(`id`, `exam`, `subjectid`, `studentid`, `remark`) VALUES (3, 79, 103, 1, '');
INSERT INTO `two20210610`.`exam`(`id`, `exam`, `subjectid`, `studentid`, `remark`) VALUES (4, 96, 104, 1, NULL);
INSERT INTO `two20210610`.`exam`(`id`, `exam`, `subjectid`, `studentid`, `remark`) VALUES (5, 85, 101, 2, NULL);
INSERT INTO `two20210610`.`exam`(`id`, `exam`, `subjectid`, `studentid`, `remark`) VALUES (6, 89, 102, 2, NULL);
INSERT INTO `two20210610`.`exam`(`id`, `exam`, `subjectid`, `studentid`, `remark`) VALUES (7, 79, 103, 2, NULL);
INSERT INTO `two20210610`.`exam`(`id`, `exam`, `subjectid`, `studentid`, `remark`) VALUES (8, 83, 104, 2, NULL);


INSERT INTO `two20210610`.`grade`(`id`, `gradename`, `major`) VALUES (20, '20级', '软件技术');


INSERT INTO `two20210610`.`studentinfo`(`StudentID`, `StudentName`, `Gender`, `Birthday`, `ClassID`, `BeginYear`, `Phone`, `Province`, `City`, `Email`) VALUES ('1', '张无忌', '男', '2021-06-10', 2001, 2020, '13012340001', '河南', '许昌', 'zwj@qq.com');
INSERT INTO `two20210610`.`studentinfo`(`StudentID`, `StudentName`, `Gender`, `Birthday`, `ClassID`, `BeginYear`, `Phone`, `Province`, `City`, `Email`) VALUES ('2', '张铁牛', '男', '2021-06-16', 2001, 2020, '13012340002', '河南', '许昌', 'ztn@qq.com');
INSERT INTO `two20210610`.`studentinfo`(`StudentID`, `StudentName`, `Gender`, `Birthday`, `ClassID`, `BeginYear`, `Phone`, `Province`, `City`, `Email`) VALUES ('3', '林平之', '男', '2021-06-15', 2001, 2020, '13012340003', '河南', '安阳', 'lpz@qq.com');
INSERT INTO `two20210610`.`studentinfo`(`StudentID`, `StudentName`, `Gender`, `Birthday`, `ClassID`, `BeginYear`, `Phone`, `Province`, `City`, `Email`) VALUES ('4', '令狐冲', '男', '2021-06-08', 2002, 2019, '13012340004', '河南', '濮阳', 'lhc@qq.com');
INSERT INTO `two20210610`.`studentinfo`(`StudentID`, `StudentName`, `Gender`, `Birthday`, `ClassID`, `BeginYear`, `Phone`, `Province`, `City`, `Email`) VALUES ('5', '岳灵珊', '女', '2021-06-02', 2002, 2019, '13012340005', '河南', '南阳', 'yls@qq.com');


INSERT INTO `two20210610`.`subject`(`id`, `subjectname`, `teacherid`) VALUES (101, 'mysql', 201);
INSERT INTO `two20210610`.`subject`(`id`, `subjectname`, `teacherid`) VALUES (102, 'java基础', 201);
INSERT INTO `two20210610`.`subject`(`id`, `subjectname`, `teacherid`) VALUES (103, '面向对象', 202);
INSERT INTO `two20210610`.`subject`(`id`, `subjectname`, `teacherid`) VALUES (104, '网页设计', 202);



INSERT INTO `two20210610`.`teacher`(`id`, `teachername`, `hiredate`, `gender`) VALUES (201, 'hys', '2021-06-04', '男');
INSERT INTO `two20210610`.`teacher`(`id`, `teachername`, `hiredate`, `gender`) VALUES (202, 'dj', '2021-06-01', '女');

用子查询实现，查询出学生“林平之”的同班同学 :

#1 找到林平之 的班级
select classid from studentinfo where studentname='林平之';

# 2 查同班同学
select*from studentinfo where classid=2001;

# 3 合并
select * from studentinfo where classid=(select classid from studentinfo where studentname='林平之');

查询《mysql》考试成绩刚好等于85分的学生名单:

-- 1 需要查询学生名单 在studentinfo
-- 2 科目是mysql  在subject表中 id
-- 3 成绩是85  在exam表中subjectid  studentid

select studentname from studentinfo inner join exam on studentinfo.StudentID=exam.studentid inner join `subject` on subject.id=exam.subjectid
where subject.subjectname='mysql' and exam.exam=85;

select studentname from studentinfo, exam,`subject`     
where studentinfo.StudentID=exam.studentid  and subject.id=exam.subjectid and subject.subjectname='mysql' and exam.exam=85;

查询《mysql》考试成绩刚好等于85分的学生名单:

-- 1 根据学生id查找学生的名单
select studentname from studentinfo where studentid=2;
-- 2 在成绩表中找到成绩为85的学生id
select studentid from exam where exam=85;
-- 3 在课程表中查询mysql的课程id
select id from subject where subjectname='mysql';

-- 融合
select studentname from studentinfo where studentid=(select studentid from exam where exam=85
and  subjectid = (select id from subject where subjectname='mysql'));

更新dj基于mtsql成绩为+5分:

-- 1 找到mysql成绩  +5  exam表  id subjectid studentid
update exam set exam = exam+5 where subjectid=101;
-- 2 找到mysql课程的id  subject表  id  subjectname 
select id from subject where subjectname='网页设计';
-- 3 根据名字找id  studentinfo 
select id from teacher where teachername='dj';
-- 融合
 update exam set exam = exam+5 where subjectid=( select id from subject where subjectname='网页设计' and
 subject.teacherid=( select id from teacher where teachername='dj'));