9 having 过滤条件 / order by 排序字段

select post,group_concat(name),count(id) from employee group by post 
having count(id)<2;

select post, avg(salary) from employee group up post 
having avg(salary)>10000 and avg(salary)<20000;

select * from employee order by age asc; #默认升序

select * from employee order by age desc; 

select * from employee order by age asc,id desc; 

select post, count(id) as emp_count from employee
where salary>500
group by post
having count(id)>1
order by count(id) desc;