04 数据库入门学习-单表查询、多表查询、子查询

1.复制表

#创建了一张测试表
mysql>create table test (id int primary key auto_increment,name char(10));

#显示原表结构
mysql> desc test;
+-------+----------+------+-----+---------+----------------+
| Field | Type     | Null | Key | Default | Extra          |
+-------+----------+------+-----+---------+----------------+
| id    | int(11)  | NO   | PRI | NULL    | auto_increment |
| name  | char(10) | YES  |     | NULL    |                |
+-------+----------+------+-----+---------+----------------+

#显示原表数据
mysql> select *from test;
+----+------+
| id | name |
+----+------+
|  1 | a    |
+----+------+


#复制表
mysql> create table copy select *from test;

#显示复制表的结构
mysql> desc copy;
+-------+----------+------+-----+---------+-------+
| Field | Type     | Null | Key | Default | Extra |
+-------+----------+------+-----+---------+-------+
| id    | int(11)  | NO   |     | 0       |       |
| name  | char(10) | YES  |     | NULL    |       |
+-------+----------+------+-----+---------+-------+

#显示复制表的数据
mysql> select *from copy;
+----+------+
| id | name |
+----+------+
|  1 | a    |
+----+------+

总结：复制表，只是拷贝结构与数据，但是索引、 描述不能拷贝（自增）

2.单表查询

2.1完整的查询语句　

#[]可选    {}必选   |或

select [distinct] {* | 字段名 | 聚合函数 | 表达式} from 表名
[where 条件
group by 字段名
having 条件
order by 字段名
limit 显示的条数]

#注意:关键字的顺序必须与上述语法一致

2.2执行顺序

1.from,先找到文件/表

2.where,拿着where的约束条件，与文件/表中的记录依次比较，正确的数据取出来

3.group，对取出来的数据进行分组

4.having,对分组的数据进行过滤

5.distinct,去重

6.order by,对筛选后的数据排序

7.limit,限制结果的显示条数

2.3简单查询

#准备表
create table stu(id int primary key auto_increment,name char(10),math float,english float); 
insert into stu values(null,"赵云",90,30); 
insert into stu values(null,"小乔",90,60); 
insert into stu values(null,"小乔",90,60); 
insert into stu values(null,"大乔",10,70); 
insert into stu values(null,"李清照",100,100); 
insert into stu values(null,"铁拐李",20,55); 
insert into stu values(null,"小李子",20,55);

mysql> select *from stu;
+----+-----------+------+---------+
| id | name      | math | english |
+----+-----------+------+---------+
|  1 | 赵云      |   90 |      30 |
|  2 | 小乔      |   90 |      60 |
|  3 | 小乔      |   90 |      60 |
|  4 | 大乔      |   10 |      70 |
|  5 | 李清照    |  100 |     100 |
|  6 | 铁拐李    |   20 |      55 |
|  7 | 小李子    |   20 |      55 |
+----+-----------+------+---------+

#避免重复
mysql> select distinct name from stu;
+-----------+
| name      |
+-----------+
| 赵云      |
| 小乔      |
| 大乔      |
| 李清照    |
| 铁拐李    |
| 小李子    |
+-----------+

#四则运算
mysql> select name,math+10 as math from stu;
+-----------+------+
| name      | math |
+-----------+------+
| 赵云      |  100 |
| 小乔      |  100 |
| 小乔      |  100 |
| 大乔      |   20 |
| 李清照    |  110 |
| 铁拐李    |   30 |
| 小李子    |   30 |
+-----------+------+

#显示格式
mysql>  select
    ->  concat("name:",name),
    ->  concat("english:",english),
    ->   concat("math:",math)
    ->   from stu;
+----------------------+----------------------------+----------------------+
| concat("name:",name) | concat("english:",english) | concat("math:",math) |
+----------------------+----------------------------+----------------------+
| name:赵云            | english:30                 | math:90              |
| name:小乔            | english:60                 | math:90              |
| name:小乔            | english:60                 | math:90              |
| name:大乔            | english:70                 | math:10              |
| name:李清照          | english:100                | math:100             |
| name:铁拐李          | english:55                 | math:20              |
| name:小李子          | english:55                 | math:20              |
+----------------------+----------------------------+----------------------+

2.4 where

1. 比较运算符：> < >= <= <> !=
2. between 80 and 100 值在80到100之间
3. in(80,90,100) 值是80或90或100
4. like 'egon[%|_]'
    %表示任意多字符
    _表示一个字符 
5. 逻辑运算符：在多个条件直接可以使用逻辑运算符 and or not

+----+-----------+------+---------+
| id | name      | math | english |
+----+-----------+------+---------+
|  1 | 赵云      |   90 |      30 |
|  2 | 小乔      |   90 |      60 |
|  3 | 小乔      |   90 |      60 |
|  4 | 大乔      |   10 |      70 |
|  5 | 李清照    |  100 |     100 |
|  6 | 铁拐李    |   20 |      55 |
|  7 | 小李子    |   20 |      55 |
+----+-----------+------+---------+


#1. 比较运算符：> < >= <= <> !=
mysql> select *from stu where math>70;
+----+-----------+------+---------+
| id | name      | math | english |
+----+-----------+------+---------+
|  1 | 赵云      |   90 |      30 |
|  2 | 小乔      |   90 |      60 |
|  3 | 小乔      |   90 |      60 |
|  5 | 李清照    |  100 |     100 |
+----+-----------+------+---------+

#2. between and 
mysql> select *from stu where math between 80 and 100 ;
+----+-----------+------+---------+
| id | name      | math | english |
+----+-----------+------+---------+
|  1 | 赵云      |   90 |      30 |
|  2 | 小乔      |   90 |      60 |
|  3 | 小乔      |   90 |      60 |
|  5 | 李清照    |  100 |     100 |
+----+-----------+------+---------+

#3. in
mysql> select *from stu where math in (80,90,100) ;
+----+-----------+------+---------+
| id | name      | math | english |
+----+-----------+------+---------+
|  1 | 赵云      |   90 |      30 |
|  2 | 小乔      |   90 |      60 |
|  3 | 小乔      |   90 |      60 |
|  5 | 李清照    |  100 |     100 |
+----+-----------+------+---------+

#4. like %或_
mysql> select *from stu where name like '小_' ;
+----+--------+------+---------+
| id | name   | math | english |
+----+--------+------+---------+
|  2 | 小乔   |   90 |      60 |
|  3 | 小乔   |   90 |      60 |
+----+--------+------+---------+

mysql> select *from stu where name like '李%' ;
+----+-----------+------+---------+
| id | name      | math | english |
+----+-----------+------+---------+
|  5 | 李清照    |  100 |     100 |
+----+-----------+------+---------+


#5. and or not
mysql> select *from stu where math=90 and english=60;
+----+--------+------+---------+
| id | name   | math | english |
+----+--------+------+---------+
|  2 | 小乔   |   90 |      60 |
|  3 | 小乔   |   90 |      60 |
+----+--------+------+---------+

2.5 group by

用于给数据分组

　　1.在生活中是为了方便管理
　　2.在数据库中是为了 方便统计

可以按照任意字段分组，但是分组完毕后，只能查看分组的字段，如果想查看组内信息，需要借助于聚合函数

聚合函数：
　　将一堆数据经过计算，得到一个数据
　　sum() 求和
　　avg() 求平均数
　　max()/min() 求最大值 / 最小值
　　count() 个数

#由于没有设置ONLY_FULL_GROUP_BY,于是也可以有结果，默认都是组内的第一条记录，但其实这是没有意义的

mysql> set global sql_mode='ONLY_FULL_GROUP_BY';
Query OK, 0 rows affected (0.00 sec)

mysql> quit #设置成功后，一定要退出，然后重新登录方可生效


#准备数据
create table emp (id int,name char(10),sex char,dept char(10),job char(10),salary double);
insert into emp values (1,"刘备","男","市场","总监",5800),
(2,"张飞","男","市场","员工",3000),
(3,"关羽","男","市场","员工",4000),
(4,"孙权","男","行政","总监",6000),
(5,"周瑜","男","行政","员工",5000),
(6,"小乔","女","行政","员工",4000),
(7,"曹操","男","财务","总监",10000),
(8,"司马懿","男","财务","员工",6000);

#查询每个部门有几个人
mysql> select dept,count(*)as 人数  from emp group by dept;
+--------+--------+
| dept   | 人数   |
+--------+--------+
| 市场   |      3 |
| 行政   |      3 |
| 财务   |      2 |
+--------+--------+

#计算每个部门的平均工资
mysql> select dept,avg(salary)as 平均工资  from emp group by dept;
+--------+-------------------+
| dept   | 平均工资          |
+--------+-------------------+
| 市场   | 4266.666666666667 |
| 行政   |              5000 |
| 财务   |              8000 |
+--------+-------------------+

2.6 having

用于对分组后的数据进行过滤
1. having不会单独出现 都是和group by 一起出现

2. where 发生在分组group by之前，因而Where中可以有任意字段，但是绝对不能使用聚合函数。

3. having发生在分组group by之后，因而Having中可以使用分组的字段，无法直接取到其他字段,可以使用聚合函数

#准备数据
mysql> create table emp (id int,name char(10),sex char,dept char(10),job char(10),salary double);

mysql> insert into emp values (1,"刘备","男","市场","总监",5800),
    -> (2,"张飞","男","市场","员工",3000),
    -> (3,"关羽","男","市场","员工",4000),
    -> (4,"孙权","男","行政","总监",6000),
    -> (5,"周瑜","男","行政","员工",5000),
    -> (6,"小乔","女","行政","员工",4000),
    -> (7,"曹操","男","财务","总监",10000),
    -> (8,"司马懿","男","财务","员工",6000);

mysql> select *from emp;
+------+-----------+------+--------+--------+--------+
| id   | name      | sex  | dept   | job    | salary |
+------+-----------+------+--------+--------+--------+
|    1 | 刘备      | 男   | 市场   | 总监   |   5800 |
|    2 | 张飞      | 男   | 市场   | 员工   |   3000 |
|    3 | 关羽      | 男   | 市场   | 员工   |   4000 |
|    4 | 孙权      | 男   | 行政   | 总监   |   6000 |
|    5 | 周瑜      | 男   | 行政   | 员工   |   5000 |
|    6 | 小乔      | 女   | 行政   | 员工   |   4000 |
|    7 | 曹操      | 男   | 财务   | 总监   |  10000 |
|    8 | 司马懿    | 男   | 财务   | 员工   |   6000 |
+------+-----------+------+--------+--------+--------+

#查询平均工资大于5000的部门
mysql> select dept,avg(salary)as 平均工资  from emp group by dept having avg(salary)>5000;
+--------+--------------+
| dept   | 平均工资     |
+--------+--------------+
| 财务   |         8000 |
+--------+--------------+


#查询每个职位有多少人
mysql> select job,count(*)as 人数  from emp group by job;
+--------+--------+
| job    | 人数   |
+--------+--------+
| 员工   |      5 |
| 总监   |      3 |
+--------+--------+

2.7 order by

用于对记录进行 排序

desc为降序
asc为升序

#准备数据
mysql> create table emp (id int,name char(10),sex char,dept char(10),job char(10),salary double);

mysql> insert into emp values (1,"刘备","男","市场","总监",5800),
    -> (2,"张飞","男","市场","员工",3000),
    -> (3,"关羽","男","市场","员工",4000),
    -> (4,"孙权","男","行政","总监",6000),
    -> (5,"周瑜","男","行政","员工",5000),
    -> (6,"小乔","女","行政","员工",4000),
    -> (7,"曹操","男","财务","总监",10000),
    -> (8,"司马懿","男","财务","员工",6000);

mysql> select *from emp;
+------+-----------+------+--------+--------+--------+
| id   | name      | sex  | dept   | job    | salary |
+------+-----------+------+--------+--------+--------+
|    1 | 刘备      | 男   | 市场   | 总监   |   5800 |
|    2 | 张飞      | 男   | 市场   | 员工   |   3000 |
|    3 | 关羽      | 男   | 市场   | 员工   |   4000 |
|    4 | 孙权      | 男   | 行政   | 总监   |   6000 |
|    5 | 周瑜      | 男   | 行政   | 员工   |   5000 |
|    6 | 小乔      | 女   | 行政   | 员工   |   4000 |
|    7 | 曹操      | 男   | 财务   | 总监   |  10000 |
|    8 | 司马懿    | 男   | 财务   | 员工   |   6000 |

#按单列排序
#默认是ASC升序
mysql> SELECT * FROM emp ORDER BY salary;
+------+-----------+------+--------+--------+--------+
| id   | name      | sex  | dept   | job    | salary |
+------+-----------+------+--------+--------+--------+
|    2 | 张飞      | 男   | 市场   | 员工   |   3000 |
|    3 | 关羽      | 男   | 市场   | 员工   |   4000 |
|    6 | 小乔      | 女   | 行政   | 员工   |   4000 |
|    5 | 周瑜      | 男   | 行政   | 员工   |   5000 |
|    1 | 刘备      | 男   | 市场   | 总监   |   5800 |
|    4 | 孙权      | 男   | 行政   | 总监   |   6000 |
|    8 | 司马懿    | 男   | 财务   | 员工   |   6000 |
|    7 | 曹操      | 男   | 财务   | 总监   |  10000 |
+------+-----------+------+--------+--------+--------+

#升序
mysql> SELECT * FROM emp ORDER BY salary ASC;
+------+-----------+------+--------+--------+--------+
| id   | name      | sex  | dept   | job    | salary |
+------+-----------+------+--------+--------+--------+
|    2 | 张飞      | 男   | 市场   | 员工   |   3000 |
|    3 | 关羽      | 男   | 市场   | 员工   |   4000 |
|    6 | 小乔      | 女   | 行政   | 员工   |   4000 |
|    5 | 周瑜      | 男   | 行政   | 员工   |   5000 |
|    1 | 刘备      | 男   | 市场   | 总监   |   5800 |
|    4 | 孙权      | 男   | 行政   | 总监   |   6000 |
|    8 | 司马懿    | 男   | 财务   | 员工   |   6000 |
|    7 | 曹操      | 男   | 财务   | 总监   |  10000 |
+------+-----------+------+--------+--------+--------+


#降序
mysql> SELECT * FROM emp ORDER BY salary DESC;
+------+-----------+------+--------+--------+--------+
| id   | name      | sex  | dept   | job    | salary |
+------+-----------+------+--------+--------+--------+
|    7 | 曹操      | 男   | 财务   | 总监   |  10000 |
|    4 | 孙权      | 男   | 行政   | 总监   |   6000 |
|    8 | 司马懿    | 男   | 财务   | 员工   |   6000 |
|    1 | 刘备      | 男   | 市场   | 总监   |   5800 |
|    5 | 周瑜      | 男   | 行政   | 员工   |   5000 |
|    3 | 关羽      | 男   | 市场   | 员工   |   4000 |
|    6 | 小乔      | 女   | 行政   | 员工   |   4000 |
|    2 | 张飞      | 男   | 市场   | 员工   |   3000 |
+------+-----------+------+--------+--------+--------+

按多列排序:

#先按照职位排序，如果职位相同，则按照薪资排序

mysql> SELECT * from emp ORDER BY job, salary DESC;
+------+-----------+------+--------+--------+--------+
| id   | name      | sex  | dept   | job    | salary |
+------+-----------+------+--------+--------+--------+
|    8 | 司马懿    | 男   | 财务   | 员工   |   6000 |
|    5 | 周瑜      | 男   | 行政   | 员工   |   5000 |
|    3 | 关羽      | 男   | 市场   | 员工   |   4000 |
|    6 | 小乔      | 女   | 行政   | 员工   |   4000 |
|    2 | 张飞      | 男   | 市场   | 员工   |   3000 |
|    7 | 曹操      | 男   | 财务   | 总监   |  10000 |
|    4 | 孙权      | 男   | 行政   | 总监   |   6000 |
|    1 | 刘备      | 男   | 市场   | 总监   |   5800 |
+------+-----------+------+--------+--------+--------+

2.8 limit

　　用于限制显示的条数
　　limit [start,]count

#准备数据
mysql> create table emp (id int,name char(10),sex char,dept char(10),job char(10),salary double);

mysql> insert into emp values (1,"刘备","男","市场","总监",5800),
    -> (2,"张飞","男","市场","员工",3000),
    -> (3,"关羽","男","市场","员工",4000),
    -> (4,"孙权","男","行政","总监",6000),
    -> (5,"周瑜","男","行政","员工",5000),
    -> (6,"小乔","女","行政","员工",4000),
    -> (7,"曹操","男","财务","总监",10000),
    -> (8,"司马懿","男","财务","员工",6000);

mysql> select *from emp;
+------+-----------+------+--------+--------+--------+
| id   | name      | sex  | dept   | job    | salary |
+------+-----------+------+--------+--------+--------+
|    1 | 刘备      | 男   | 市场   | 总监   |   5800 |
|    2 | 张飞      | 男   | 市场   | 员工   |   3000 |
|    3 | 关羽      | 男   | 市场   | 员工   |   4000 |
|    4 | 孙权      | 男   | 行政   | 总监   |   6000 |
|    5 | 周瑜      | 男   | 行政   | 员工   |   5000 |
|    6 | 小乔      | 女   | 行政   | 员工   |   4000 |
|    7 | 曹操      | 男   | 财务   | 总监   |  10000 |
|    8 | 司马懿    | 男   | 财务   | 员工   |   6000 |
+------+-----------+------+--------+--------+--------+




# 看看表里前三条数据
mysql> select *from emp limit 3;
+------+--------+------+--------+--------+--------+
| id   | name   | sex  | dept   | job    | salary |
+------+--------+------+--------+--------+--------+
|    1 | 刘备   | 男   | 市场   | 总监   |   5800 |
|    2 | 张飞   | 男   | 市场   | 员工   |   3000 |
|    3 | 关羽   | 男   | 市场   | 员工   |   4000 |
+------+--------+------+--------+--------+--------+

# 看看表里的3-5条
mysql> select *from emp limit 2,3;
+------+--------+------+--------+--------+--------+
| id   | name   | sex  | dept   | job    | salary |
+------+--------+------+--------+--------+--------+
|    3 | 关羽   | 男   | 市场   | 员工   |   4000 |
|    4 | 孙权   | 男   | 行政   | 总监   |   6000 |
|    5 | 周瑜   | 男   | 行政   | 员工   |   5000 |
+------+--------+------+--------+--------+--------+

# 查看工资最高的那个人的信息
mysql> select *from emp order by salary desc  limit 1;
+------+--------+------+--------+--------+--------+
| id   | name   | sex  | dept   | job    | salary |
+------+--------+------+--------+--------+--------+
|    7 | 曹操   | 男   | 财务   | 总监   |  10000 |
+------+--------+------+--------+--------+--------+

3.多表查询

在多个表中查询需要的数据
例如：有班级表和学生表
　　给你一个班级名称， 请查询所有的学员数据
　　先查班级表 得到一个班级的id， 再根据id去学院表查询对应的学员

#准备数据：
create table emp (id int,name char(10),sex char,dept_id int);
insert emp values(1,"大黄","m",1);
insert emp values(2,"老王","m",2);
insert emp values(3,"老李","w",30);
mysql> select *from emp;
+------+--------+------+---------+
| id   | name   | sex  | dept_id |
+------+--------+------+---------+
|    1 | 大黄   | m    |       1 |
|    2 | 老王   | m    |       2 |
|    3 | 老李   | w    |      30 |
+------+--------+------+---------+


create table dept (id int,name char(10));
insert dept values(1,"市场");
insert dept values(2,"财务");
mysql> select *from dept;
+------+--------+
| id   | name   |
+------+--------+
|    1 | 市场   |
|    2 | 财务   |
|    3 | 行政   |
+------+--------+
insert dept values(3,"行政");

 多表查询的方式

1.笛卡尔积查询

#什么是笛卡尔积,用坐标中的一条记录，去链接另一张表的所有记录
#就像是把两张表的数据做了一个乘法
#这将导致产生大量的无用重复数据
#我们要的效果是：员工表中的部门id与部门表中的id相同，就拼接在一起
#用 where 筛选出正确的数据
mysql> select *from emp,dept where emp.dept_id=dept.id;
+------+--------+------+---------+------+--------+
| id   | name   | sex  | dept_id | id   | name   |
+------+--------+------+---------+------+--------+
|    1 | 大黄   | m    |       1 |    1 | 市场   |
|    2 | 老王   | m    |       2 |    2 | 财务   |
+------+--------+------+---------+------+--------+

2.内连接查询

#inner  join...on... 
mysql> select *from emp inner join dept on emp.dept_id=dept.id;
+------+--------+------+---------+------+--------+
| id   | name   | sex  | dept_id | id   | name   |
+------+--------+------+---------+------+--------+
|    1 | 大黄   | m    |       1 |    1 | 市场   |
|    2 | 老王   | m    |       2 |    2 | 财务   |
+------+--------+------+---------+------+--------+
#与from...where...一样，只不过inner  join...on...是专门用于多表连接的

3.左外连接

left join
#左边员工表中的数据完全显示，右边表中的数据匹配上才显示，没有则NULL
mysql> select *from emp left  join dept on emp.dept_id=dept.id;
+------+--------+------+---------+------+--------+
| id   | name   | sex  | dept_id | id   | name   |
+------+--------+------+---------+------+--------+
|    1 | 大黄   | m    |       1 |    1 | 市场   |
|    2 | 老王   | m    |       2 |    2 | 财务   |
|    3 | 老李   | w    |      30 | NULL | NULL   |
+------+--------+------+---------+------+--------+

4.右外连接

right join
#右边表中的数据完全显示，左边表中的数据匹配上才显示 
mysql> select *from emp right  join dept on emp.dept_id=dept.id;
+------+--------+------+---------+------+--------+
| id   | name   | sex  | dept_id | id   | name   |
+------+--------+------+---------+------+--------+
|    1 | 大黄   | m    |       1 |    1 | 市场   |
|    2 | 老王   | m    |       2 |    2 | 财务   |
| NULL | NULL   | NULL |    NULL |    3 | 行政   |
+------+--------+------+---------+------+--------+

5. 全外链接

#全外连接：在内连接的基础上增加左边有右边没有的和右边有左边没有的结果
#mysql不支持 full join，但是可以用union间接实现

union：表示合并查询，意思是把多个查询结果合并在一起显示，要求是被合并的表结构必须相同，默认去除重复

union all：合并但是不去除重复

sql> select *from emp right join dept  on emp.dept_id = dept.id
    ->union
    ->select *from emp left join dept  on emp.dept_id = dept.id;
+------+--------+------+---------+------+--------+
| id   | name   | sex  | dept_id | id   | name   |
+------+--------+------+---------+------+--------+
|    1 | 大黄   | m    |       1 |    1 | 市场   |
|    2 | 老王   | m    |       2 |    2 | 财务   |
| NULL | NULL   | NULL |    NULL |    3 | 行政   |
|    3 | 老李   | w    |      30 | NULL | NULL   |
+------+--------+------+---------+------+--------+


                        

6.三表查询

#准备数据
#老师表
create table tec(id int,name char(10));
insert into tec value(1,"egon");
insert into tec value(2,"yyh");
mysql> select *from tec;
+------+------+
| id   | name |
+------+------+
|    1 | egon |
|    2 | yyh  |
+------+------+

#学生表
create table stu(id int,name char(10));
insert into stu value(1,"大傻");
insert into stu value(2,"中傻");
insert into stu value(3,"小傻");
mysql> select *from stu;
+------+--------+
| id   | name   |
+------+--------+
|    1 | 大傻   |
|    2 | 中傻   |
|    3 | 小傻   |


#关联表
create table s_t(s_id int,t_id int);
insert into s_t value(1,2);
insert into s_t value(2,2);
insert into s_t value(3,1);
mysql> select *from s_t;
+------+------+
| s_id | t_id |
+------+------+
|    1 |    2 |
|    2 |    2 |
|    3 |    1 |
+------+------+

#需求  找出 yyh 这个老师 教过的学生信息
#思路：
#    第一步，到关系表中去查询，哪些老师教过哪些学生(学生的id)，形成了一个临时表
mysql> select *from tec inner join s_t on tec.id=s_t.t_id;
+------+------+------+------+
| id   | name | s_id | t_id |
+------+------+------+------+
|    2 | yyh  |    1 |    2 |
|    2 | yyh  |    2 |    2 |
|    1 | egon |    3 |    1 |
+------+------+------+------+


#    第二步， 将上一步得到临时表与学生表进行连接
mysql> select *from tec inner join s_t on tec.id=s_t.t_id
    -> inner join stu on s_id=stu.id;
+------+------+------+------+------+--------+
| id   | name | s_id | t_id | id   | name   |
+------+------+------+------+------+--------+
|    2 | yyh  |    1 |    2 |    1 | 大傻   |
|    2 | yyh  |    2 |    2 |    2 | 中傻   |
|    1 | egon |    3 |    1 |    3 | 小傻   |
+------+------+------+------+------+--------+


#    第三步，加上额外的筛选条件，老师的name是yyh
mysql> select tec.name as 老师名字  ,stu.name as 学生名字  from tec inner join s_t on tec.id=s_t.t_id
    -> inner join stu on s_id=stu.id
    -> where tec.name='yyh';
+--------------+--------------+
| 老师名字     | 学生名字     |
+--------------+--------------+
| yyh          | 大傻         |
| yyh          | 中傻         |
+--------------+--------------+

4.子查询

子查询：将上一次查询的结果作为本次查询的原始数据（或是查询条件）

#准备数据
create table emp (id int,name char(10),sex char,dept char(10),job char(10),salary double);
insert into emp values (1,"刘备","男","市场","总监",5800),
(2,"张飞","男","市场","员工",3000),
(3,"关羽","男","市场","员工",4000),
(4,"孙权","男","行政","总监",6000),
(5,"周瑜","男","行政","员工",5000),
(6,"小乔","女","行政","员工",4000),
(7,"曹操","男","财务","总监",10000),
(8,"司马懿","男","财务","员工",6000);

#需求：查询出工资最高的人的信息
#先查询出最高工资是多少
#拿着最高工资去表中看，谁的工资和最高工资匹配

mysql> select *from emp where salary=(select max(salary) from emp);
+------+--------+------+--------+--------+--------+
| id   | name   | sex  | dept   | job    | salary |
+------+--------+------+--------+--------+--------+
|    7 | 曹操   | 男   | 财务   | 总监   |  10000 |
+------+--------+------+--------+--------+--------+
#in 关键字子查询
#查询平均年龄大于25的部门名称
#1.求每个部门的平均年龄
select avg(age) from emp group by dept_id;
#2.筛选结果
select dept_id,avg(age) as a from emp group by dept_id having a > 25;
#3.最后
select name from dept where id in (select dept_id from emp group by dept_id having avg(age) > 25);



#子查询方式:
#平均年龄大于25的部门有哪些?
#先要求出每个部门的平年龄!每个表示什么? 分组
select name from dept where id in  (select dept_id from emp group by dept_id having avg(age) > 25);

#多表查询方式:
#先把数据拼接到一起 在加以筛选
select dept.name from emp inner join dept
on emp.dept_id = dept.id 
group by dept.name
having avg(age) >25;

#exists关键字子查询
#exists  后跟子查询  子查询有结果是为True 没有结果时为False
#为true时外层执行 为false外层不执行
select *from emp where exists (select *from emp where salary > 1000);
select (exists (select *from emp where salary > 10000));