mysql的分组查询

1.设置数据库为严格模式：

2.数据准备

# 创建一张部门表
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
);


# 插入记录
# 三个部门：教学，销售，运营
insert into emp(name,sex,age,hire_date,post,salary,office,depart_id) values
('tank','male',17,'20170301','张江第一帅形象代言部门',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),
('jason','male',73,'20140701','teacher',3500,401,1),
('owen','male',28,'20121101','teacher',2100,401,1),
('jerry','female',18,'20110211','teacher',9000,401,1),
('大饼','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
```

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完整的查询语句：

 完整的查询语句
    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]

查询顺序：
    - from
    - where
    - group by
    - having
    - select 
    - distinct

3 . where 约束

# 1.查询id大于等于3小于等于6的数据
select * from emp where id >= 3 and id <= 6;
# between: 两者之间
# and: 与
select * from emp where id between 3 and 6;


# 2.查询薪资是20000或者18000或者17000的数据
# or:  或者
select * from emp where salary=20000 or salary=18000 or salary=17000;
# in: 在什么里
select * from emp where salary in (20000, 18000, 17000);


# 3.查询员工姓名中包含o字母 的 员工姓名和薪资
# like: 模糊匹配
# %: 匹配0个或多个任意字符   以什么开头 以什么结尾 包含什么
# _: 匹配一个任意字符       ___匹配任意的三个字符
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与薪资
select name, salary from emp where name like "_____";
# char_length(字段): 获取字段长度
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名与岗位名  
# 注意: 针对null不能用等号，只能用is     is null
select name, post from emp where post_comment = null;
select name, post from emp where post_comment is null;
select name,post from emp where post_comment is not null;
```

4. group by 聚合函数: max, min, sum, avg, count

比如: 一张员工表中有性别字段，可以根据性别分组，一组是男性，一组是女性，或者是根据部门分组，有教学部、销售部等...

# 1.按部门分组
# 严格模式下只能获取分组字段post数据, 无法获取其他字段信息，就好比是进程之间数据隔离，但是可以使用聚合函数来获取
'''
聚合函数: max, min, sum, avg, count
'''
# 非严格模式下不报错
select * from emp group by post;  # 报错
select id, name from emp group by post;  # 报错
select post from emp group by post;

# 严格模式设置
"""
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
"""
show variables like "%mode%";
set global sql_mode="strict_trans_tables,only_full_group_by";


# 2.获取每个部门的最高工资 ----> 画图，讲述数据分组后，发生了什么事情
select post, max(salary) from emp group by post;
# as: 起别名; 给获取出来的数据字段名，设置别名
select post as '部门', max(salary) as '薪资' from emp group by post;
# 可简写, 但不推荐
select post '部门', max(salary) '薪资' 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: count() 中传任意参数都没问题
select post, count(id) from emp group by post;
select post, count(post) 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员工的姓名
# group_concat(name): 不仅可以获取分组后的某一个字段，并且可以对字符串进行拼接
select post, group_concat(name) from emp group by post;
# 给每个部门的员工名字前 + NB_
select post, group_concat('NB_', name) from emp group by post;
# 拼接部门员工名字+薪资
select post, group_concat(name, ":", salary) from emp group by post;


# 4.补充concat（不分组时用）拼接字符串达到更好的显示效果 as语法使用
select concat('Name: ', name) as '名字', concat('Sal: ', salary) as '薪资' from emp;


# 5.补充as语法 即可以给字段起别名也可以给表起
select emp.name as '名字', emp.salary as '薪资' from emp;


# 6.查询四则运算
# 求各部门所有员工的年薪
select name, salary * 12 as annual_salary from emp;
```

5.having 语句

where 与 having的区别：

1. having与where语法一样，只不过having必须需要在group by后使用;

2. where 不能使用聚合函数，但having可以;

如 where avg(salary) >1000 是错误的 having avg(salary) > 1000

6. distinct : 去重

select distinct id, post from emp;
select distinct post from emp;
select distinct age from emp;

7.order by

# 1、根据薪资进行升序
select * from emp order by salary;  # 默认升序
select * from emp order by salary asc;  # 指定升序
select * from emp order by salary desc;  # 指定降序

8. limit

# 1、从第一条开始，获取4条记录;
select * from emp limit 4;    # 查询前4行

# 2、limit可以有两个参数, 参数1:是限制的开始位置, 参数2:是从开始位置展示的条数;
select * from emp limit 0, 4;   
select * from emp limit 4, 4;  #  获取5-8行

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