单表查询是什么

select distinct 字段1，字段2... from 表名
	where 分组之前的过滤条件
	group by 分组字段
	having 分组之后的过滤条件
	order by 排序字段
	limit 显示的条数;

emp表：
员工id      id                  int             
姓名        emp_name            varchar
性别        sex                 enum
年龄        age                 int
入职日期    hire_date           date
岗位        post                varchar
职位描述    post_comment        varchar
薪水        salary              double
办公室      office              int
部门编号    depart_id           int	

create table emp(
id int not null unique auto_increment,
name varchar(20) not null,
sex enum('male','female') not null default 'male', 
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, 
depart_id int
);

insert into emp(name,sex,age,hire_date,post,salary,office,depart_id) values
('niange','male',23,'20170301','manager',15000,401,1), 
('monicx','male',23,'20150302','teacher',16000,401,1),
('wupeiqi','male',25,'20130305','teacher',8300,401,1),
('yuanhao','male',34,'20140701','teacher',3500,401,1),
('anny','female',48,'20150311','sale',3000.13,402,2),
('monke','female',38,'20101101','sale',2000.35,402,2),
('sandy','female',18,'20110312','sale',1000.37,402,2),
('chermy','female',18,'20130311','operation',19000,403,3),
('bailes','male',18,'20150411','operation',18000,403,3),
('omg','female',18,'20140512','operation',17000,403,3);

set global sql_mode="strict_trans_tables,only_full_group_by";

每个部门的最高工资
select post,max(salary) from emp group by post;
每个部门的最底工资
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;
每个部门的工资总合
select post,sum(salary) from emp group by post;
每个部门的总人数
select post,count(id) from emp group by post;

select post,group_concat(name) from emp group by post;
select post,group_concat(name,"_NB") from emp group by post;
select post,group_concat(name,": ",salary) from emp group by post;
select post,group_concat(salary) from emp group by post;

 # 补充as语法
    mysql> select emp.id,emp.name from emp as t1; # 报错
    mysql> select t1.id,t1.name from emp as t1;

    1. 查询岗位名以及岗位包含的所有员工名字
        select post,group_concat(name) from emp group by post;
    2. 查询岗位名以及各岗位内包含的员工个数
        select post,count(id) from emp group by post;
    3. 查询公司内男员工和女员工的个数
        select sex,count(id) from emp group by sex;
    4. 查询岗位名以及各岗位的平均薪资
        select post,avg(salary) from emp group by post;
    5. 查询岗位名以及各岗位的最高薪资
        select post,max(salary) from emp group by post;
    6. 查询岗位名以及各岗位的最低薪资
	select post,min(salary) from emp group by post;
    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
        select sex,avg(salary) from emp group by sex;
    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;

select * from emp order by salary asc; #默认升序排
select * from emp order by salary desc; #降序排
select * from emp order by age desc; #降序排
select * from emp order by age desc,salary asc; #先按照age降序排，再按照薪资升序排