Mysql系列课程--第五章高级查询表连接子查询 case when

一.表连接

1.内连接

1.1等值内连接
/李文辉是哪个班的/

select s.s_name,c.c_name from student s inner join class c on s.c_no = c.c_no
 where s.s_name='李文辉';（第一种写法）
select s.s_name,c.c_name from student s,class c where s.c_no = c.c_no
 and s.s_name='李文辉';（第二种写法）

/李文辉报考了哪些门课/

select s_name,sb_name from student s,result r ,subject sb where s.s_no=r.s_no and 
r.sb_no = sb.sb_no and s.s_name='李文辉';（第一种写法）
select s_name,sb_name from (student s inner join result r on s.s_no = r.s_no)inner 
join subject sb on r.sb_no = sb.sb_no where  s.s_name='李文辉';（第二种写法）

/每一门课有多少人报考/

select sb_name,count(*) from student s,result r ,subject sb where s.s_no=r.s_no 
and r.sb_no = sb.sb_no group by sb_name;

1.2不等值内连接

select s.s_name,c_name  from  student s,class c
where not s.c_no > c.c_no and not s.c_no < c.c_no  and  s.s_name='李文辉';

2.外连接

2.1左外连接

/查看各门课的成绩/

select sb_name,score from subject s left join result r on s.sb_no = r.sb_no;

/各老师的代课平均分/

select t_name,avg(score) from teacher t left join subject sb on t.t_no 
= sb.t_no left join result r on r.sb_no = sb.sb_no group by t_name

2.2 右外连接

/学生的分班情况/

select s_name,c_name from class c right join student s  on s.c_no = c.c_no;

二.子查询

1.单行子查询

/李文辉在哪个班/

select c.c_name from class c where c_no = (select c_no from student  where s_name 
= '李文辉');

/极客巅峰有哪些学生/

select s_name from student where c_no = (select c_no from class where c_name='极客巅峰');

/查询出年龄最大的学生/

select s_name from student where s_age = (select max(s_age) from student);

2.多行子查询
/查询大于5人的班级学生信息/

select * from student where c_no in (select c_no from class where c_num>5);

/查询比女生年龄都大的男生/

select * from student where s_age > ALL (select s_age from student where s_sex='女') 
and s_sex = '男';

/查询比女生年龄大的男生/

select * from student where s_age > ANY (select s_age from student where s_sex='女') 
and s_sex = '男';

2.多列子查询

/查询年龄大于25的男生/（不要在意这些细节，知识点重要）

select * from student where (s_sex,s_age) in (select s_sex,s_age from student 
where s_sex='男' and s_age>25);

3.嵌套子查询

/嵌套在where后面/

select * from student where c_no = (select c_no from class where c_name='极客巅峰');

/嵌套在from后面/（不要在意这些细节，知识点重要）

select * from (select s_name from student) s;

/嵌套在select后面/（不要在意这些细节，知识点重要）

select (select s_name from student where s_name='李文辉') from student where s_name='李文辉';

/极客巅峰JAVA课程谁考了第一/

select s_name,sb_name,score from student s,class c,subject sb,result r where s.c_no
=c.c_no and sb.sb_no = r.sb_no and r.s_no=s.s_no and c.c_name='极客巅峰' and sb.sb_name
='java' and r.score=(select max(score) from result r,subject sb where r.sb_no = sb.sb_no
 and sb.sb_name='java' );

/每门学科的状元(第一名)/

select s_name,sb_name,score from student s,result r,subject sb where s.s_no = r.s_no and 
r.sb_no = sb.sb_no and (sb_name,score) in (select sb_name,max(score) m from result r ,
subject sb where r.sb_no = sb.sb_no group by sb.sb_name);(第一种写法)

select s_name,sb_name,score from student s,result r,subject sb,(select sb_name sn,
max(score) ms from result r ,subject sb where r.sb_no = sb.sb_no group by sb.sb_name) m
 where s.s_no = r.s_no and r.sb_no = sb.sb_no and sb.sb_name = m.sn and m.ms = r.score;
 (第二种写法)

/各班年龄最大的学生/

select s_name,c_name,s_age  from student s,class c where s.c_no = c.c_no and 
(c_name,s_age) in (select c_name,max(s_age) from student s,class c 
where s.c_no = c.c_no group by c.c_name)

三. case when用法

/男女性别互换/

select s_name,case s_sex when '男' then '女' when '女' then '男'  end   
from student;

/学生各门课的考试等级/

select s_name,sb_name,score,case when score>90 then 'A' when score>80 then 'B'  
when score>70 then 'C' else 'D'   end  '等级' from result r,student s,subject sb 
where r.s_no = s.s_no and r.sb_no = sb.sb_no;

详细课程查询：
Mysql系列课程–第一章 Sql分类常用命令
 Mysql系列课程–第二章约束自增主键
 Mysql系列课程–第三章建表插数据
 Mysql系列课程–第四章备份普通查询
 Mysql系列课程–第五章高级查询表连接子查询 case when
Mysql系列课程–第六章索引和视图
 Mysql系列课程–第七章触发器和存储过程
 Mysql系列课程–第八章 sql精选35道题

Mysql系列课程--第五章 高级查询 表连接 子查询 case when

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