## **单表查询**
## <blockquote class="danger"><p>表创建</p></blockquote>
```
create table emp(
id int not null unique auto_increment,
name varchar(20) not null,
sex enum('male','female') not null default 'male', #大部分是男的
age int(3) unsigned not null default 28,
hire_date date not null,
post varchar(50),
post_comment varchar(100),
salary double(15,2),
office int, #一个部门一个屋子
depart_id int
);
```
## <blockquote class="danger"><p>插入记录【三个部门：教学，销售，运营】</p></blockquote>
insert into emp(name,sex,age,hire_date,post,salary,office,depart_id) values
('jason','male',18,'20170301','张江第一帅形象代言',7300.33,401,1), #以下是教学部
('egon','male',78,'20150302','teacher',1000000.31,401,1),
('kevin','male',81,'20130305','teacher',8300,401,1),
('tank','male',73,'20140701','teacher',3500,401,1),
('owen','male',28,'20121101','teacher',2100,401,1),
('jerry','female',18,'20110211','teacher',9000,401,1),
('nick','male',18,'19000301','teacher',30000,401,1),
('sean','male',48,'20101111','teacher',10000,401,1),
('歪歪','female',48,'20150311','sale',3000.13,402,2),#以下是销售部门
('丫丫','female',38,'20101101','sale',2000.35,402,2),
('丁丁','female',18,'20110312','sale',1000.37,402,2),
('星星','female',18,'20160513','sale',3000.29,402,2),
('格格','female',28,'20170127','sale',4000.33,402,2),
('张野','male',28,'20160311','operation',10000.13,403,3), #以下是运营部门
('程咬金','male',18,'19970312','operation',20000,403,3),
('程咬银','female',18,'20130311','operation',19000,403,3),
('程咬铜','male',18,'20150411','operation',18000,403,3),
('程咬铁','female',18,'20140512','operation',17000,403,3)
;
#ps：如果在windows系统中，插入中文字符，select的结果为空白，可以将所有字符编码统一设置成gbk
## <blockquote class="danger"><p>查询记录</p></blockquote>
1. 语法执行顺序
1. 初识查询语句 select id,name from emp where id \>= 3 and id <= 6;
先后顺序 from where select
2. where约束条件
## 1.查询id大于等于3小于等于6的数据
select id,name from emp where id >=3 and id <= 6;
select * from emp where id between 3 and 6;
## 2.查询薪资是20000或18000或17000的数据
select * from emp where salary = 20000 or salare = 18000 or salary = 17000;
select * from emp where salary in (20000,18000,17000); # 简写
## 3.查询员工姓名中包含字母o的员工姓名和薪资
在刚开始接触mysql查询时，建议按照查询的优先级顺序拼写出你的sql语句
"""
先是查哪张表 from emp
再是根据什么条件去查 where name like ‘%o%’
再是对查询出来的数据筛选展示部分 select name,salary
select name,salary from emp where name like '%o%';
## 4.查询员工姓名是由四个字符组成的员工姓名和其薪资
select name,salary from emp where name like '____';
select name,salary from emp where char_length(name) = 4;
## 5.查询id小于3或者大于6的数据
select * from emp where id <3 or id >6;
select * from emp where id not between 3 and 6;
## 6.查询薪资不在20000,18000,17000范围的数据
select * from emp where salary not in (20000,18000,17000);
## 7.查询岗位描述为空的员工姓名与岗位名 针对null不能用等号，只能用is
select name,post from emp where post_comment = null; # 查询为空！
select name,post from emp where post_comment is null;
select name,post from emp where post_comment is not null;
3. group by
## 数据分组应用场景：每个部门的平均薪资，男女比例等
## 1.按部门分组
select * from emp group by post; # 分组后取出的是每一个组的第一条数据
select id,name,sex from emp group by post # 验证
"""
设置sql_mode为only_full_group_by，意味着以后但凡分组，只能取到分组的依据，
不应该在去取组里面的单个元素的值，那样的话分组就没有意义了，因为不分组就是对单个元素信息的随意获取
"""
set global sql_mode="strict_trans_tables,only_full_group_by";
## 重新链接客户端
select * from emp group by post; # 报错
select id,name,sex from emp group by post; # 报错
select post from emp group bty post; # 获取部门信息
## 强调：只要分组了，就不能够再"直接"查找到单个数据信息了，只能获取到组名
## 2.获取每个部门的最高工资
## 以组为单位统计组内数据>>>聚合查询(聚集到一起合为一个结果)
## 每个部门的最高工资
select post,max(salary) from emp group by post;
## 每个部门的最低工资
select post,min(salary) from emp group by post;
## 每个部门的平均工资
select post,avg(salary) from emp group by post;
## 每个部门的工资总和
select post,sum(salary) from emp group by post;
## 每个部门的人数
select post,count(id) from emp group by post;
## 3.查询分组之后的部门名称和每个部门下所有学生姓名
## group_concat(分组之后用)不仅可以用来显示除分组外字段还有拼接字符串的作用
select post,group_concat(name) from emp group by post;
select post,group_concat(name,"_SB") from emp group by post;
select post,group_concat(name,"：") from emp group by post;
select post,group_concat(salary) from emp group by post;
## 4.补充concat(不分组是用)拼接字符串达到更好的显示效果 as语法使用
select name as 姓名,salary as 薪资 from emp;
select concat("NAME:",name) as 姓名,concat("SAL:",salary) as 薪资 from emp;
## 补充 as 语法,即可以给字段起别名也可以给表起
select emp.id,emp.name from emp as t1; # 报错 因为表名已经被你改成了t1
select t1.id,t1.name from emp as t1;
## 查询四则运算
## 查询每个人的年薪
select name,salary*12 as annual_salary from emp;
select
name,salary*12 annual_salary from emp; # as可以省略
4. having
select 查询字段1,查询字段2,... from 表名
where 过滤条件
group by分组依据
## 语法这么写，但是执行顺序却不一样
from
where
group by
select
5. distinct
1. 对有重复的展示数据进行去重操作 ```select distinct post from emp;```
6. order by
1. 默认升序排 ```select * from emp order by salary asc; ```
2. 降序排 ```select * from emp order by salary desc;```
3. 先按照age降序排，在年轻相同的情况下再按照薪资升序排 ```select * from emp order by age desc,salary asc;```
4. 统计各部门年龄在10岁以上的员工平均工资，并且保留平均工资大于1000的部门，然后对平均工资进行排序 ```select post,avg(salary) from emp where age > 10 group by post having avg(salary) > 1000 order by avg(salary) ;```
7. limit
1. 限制展示条数 ```select * from emp limit 3; ```
2. 查询工资最高的人的详细信息 ```select * from emp order by salary desc limit 1; ```
3. 分页显示 ```select * from emp limit 0,5;```
8. 正则 regexp
命令：``select * from emp where name regexp '^j.\*(n|y)$'; ```
9. 练习
## 刚开始查询表，一定要按照最基本的步骤，先确定是哪张表，再确定查这张表也没有限制条件，再确定是否需要分类，最后再确定需要什么字段对应的信息
1. 查询岗位名以及岗位包含的所有员工名字
2. 查询岗位名以及各岗位内包含的员工个数
3. 查询公司内男员工和女员工的个数
4. 查询岗位名以及各岗位的平均薪资
5. 查询岗位名以及各岗位的最高薪资
6. 查询岗位名以及各岗位的最低薪资
7. 查询男员工与男员工的平均薪资，女员工与女员工的平均薪资
"""
参考答案：
select post,group_concat(name) from emp group by post;
select post,count(id) from emp group by post;
select sex,count(id) from employee group by sex;
select post,avg(salary) from emp group by post;
select post,max(salary) from employee group by post;
select post,min(salary) from employee group by post;
select sex,avg(salary) from employee group by sex;
"""
## 关键字where group by同时出现的情况下，group by必须在where之后
## where先对整张表进行一次筛选，如何group by再对筛选过后的表进行分组
## 如何验证where是在group by之前执行而不是之后 利用聚合函数 因为聚合函数只能在分组之后才能使用
select id,name,age from emp where max(salary) > 3000; # 报错！
select max(salary) from emp;
## 正常运行，不分组意味着每一个人都是一组，等运行到max(salary)的时候已经经过where，group by操作了，只不过我们都没有写这些条件
## 语法顺序
select
from
where
group by
## 再识执行顺序
from
where
group by
select
8、统计各部门年龄在30岁以上的员工平均工资
select post,avg(salary) from emp where age > 30 group by post;
## 对where过滤出来的虚拟表进行一个分组
## 还不明白可以分步执行查看结构
select * from emp where age>30;
## 基于上面的虚拟表进行分组
select * from emp where age>=30 group by post;
