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));
