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+# 1. 学习
+
+```sql
+select 字段(子查询) from 表
+
+select 字段 from 条件 (子查询)
+
+select 字段 from (子查询) as 别名
+```
+
+
+
+# 2. 作业
+
+```sql
+-- 在如图的数据表上完成以下题目
+-- 
+create database xzx charset utf8;
+use xzx;
+create table stuinfo(
+	stuno varchar(10) primary key,
+	stuname varchar(10),
+	stusex enum('男','女'),
+	stuage INT,
+	stuaddress varchar(100),
+	stuseat int 
+);
+
+create table stuexam(
+	examno int primary key,
+	stuno varchar(10),
+	writtenexam int,
+	labexam int
+);
+
+create table stumarks(
+	examno int primary key,
+	stuid varchar(10),
+	score  int
+);
+alter table stuexam add foreign key (stuno) references stuinfo(stuno);
+
+alter table stumarks add foreign key(examno) references stuexam(examno) on update cascade on delete cascade;
+
+alter table stumarks add foreign key(stuid) references stuinfo(stuno) on update cascade on delete cascade;
+
+
+select * from stuinfo;
+desc stuinfo;
+insert into stuinfo values('s2501','张秋利','男',20,'美国硅谷',1),('s2502','李斯文','女',18,'湖北武汉',2),('s2503','马文才','男',18,'湖南长沙',3),('s2504','欧阳俊雄','女',21,'湖北武汉',4),('s2505','梅超风','男',20,'湖北武汉',5),('s2506','陈旋风','男',19,'美国硅谷',6);
+
+desc stuexam;
+insert into stuexam values(1,'s2501',50,70),(2,'s2502',60,65),(3,'s2503',86,70),(4,'s2504',40,80),(5,'s2505',70,85),(6,'s2506',85,90);
+
+desc stumarks;
+insert into stumarks values(1,'s2501',88),(2,'s2501',92),(3,'s2501',53),(4,'s2502',60),(5,'s2502',99),(6,'s2503',82);
+
+-- 1.查询出年龄比班上平均年龄大的学生的信息
+		select * from stuinfo where stuage > (select avg(stuage) from stuinfo);
+-- 2.查询出每个学生的学号，姓名，性别和选修课程的最高分（stuMarks）
+		select x.stuno,x.stuname,stusex,max(score) from stuinfo x left join stumarks y on x.stuno=y.stuid GROUP BY stuname;
+-- 3.查询出每个学生的学号，姓名，性别和考试平均分（stuExam）
+		select x.stuno,x.stuname,x.stusex,(writtenexam+labexam)/2 平均分 from stuinfo x inner join stuexam y on x.stuno=y.stuno;
+-- 
+-- 4.查询性别是男并且年龄大于等于20的学生的信息(用两种方法实现：普通查询和子查询)	
+	select * from stuinfo where stusex='男' and stuage>=20;
+	select * from  (select * from stuinfo where stusex='男' and stuage>=20) a;
+-- 5.查询出年龄比所有男生年龄都大的女生的信息
+    select * from stuinfo where stuage>(select max(stuage) from stuinfo where stusex='男')
+-- 6.查询出所有选修课程都及格的学生的信息   （stuMarks）
+	select * from stuinfo x left join stumarks y on x.stuno=y.stuid where stuid  not in (select stuid from stumarks where score <60) GROUP BY stuname;
+-- 7.查询出参加考试的学生的信息（用表连接，in二种方法做）（stuMarks）
+	select * from stuinfo inner join stumarks on stuinfo.stuno=stumarks.stuid GROUP BY stuname;
+	select * from stuinfo where stuno in(select stuid from stumarks);
+-- 8.查询出没有参加考试的学生的信息（用表连接，in二种方法做）（stuMarks）
+	select * from stuinfo left join stumarks on stuinfo.stuno=stumarks.stuid where stuid is null;
+	select * from stuinfo where stuno NOT in (select stuid from stumarks);
+-- 9.将有一门成绩成绩大于90分的学生的基本信息查询出来（stuMarks）
+	select * from stuinfo inner join stumarks on stuinfo.stuno=stumarks.stuid where score>90;
+-- 10.查询出平均成绩在80分以上的学生的基本信息（stuMarks）
+    select * from stuinfo inner join stumarks on stuinfo.stuno=stumarks.stuid where score>(select avg(score)     from stumarks);
+-- 
+-- 11.查询出某同学所有考试成绩比“张秋利”同学所有分数都高的学生基本信息（stuMarks）
+	select * from stuinfo inner join stumarks on stuinfo.stuno=stumarks.stuid where score > (select             max(score) from stumarks where stuid=(select stuno from stuinfo where stuname='张秋利'));
+-- 
+-- 12.查询出某同学所有考试成绩只需要比“张秋利”同学某个分数高的学生基本信息（stuMarks）
+	select * from stuinfo inner join stumarks on stuinfo.stuno=stumarks.stuid where score > any (select         score from stumarks where stuid=(select stuno from stuinfo where stuname='张秋利')) and stuname!='张秋利';
+-- 
+-- 13.查询班上比所有男生年龄都要大的女生的信息
+	select * from stuinfo where stuage>(select max(stuage) from stuinfo where stusex='男')
+-- 14.查询出只是比某个男生年龄大的女生的信息
+	select * from stuinfo where stuage >= any (select stuage from stuinfo where stusex='男') and                 stusex!='男';
+-- -- 
+```
+