聚合函数

max() 求最大值

min() 求最小值

avg() 求平均值

sum() 求和

count() 求总个数

having 分组之后筛选条件的 一般跟聚合函数连用 

order by 排序

desc 降序

asc 升序

select * from employee order by salary desc;

limit 取限定条数

select * from employee order by salary desc limit 2; 开头取两条

select * from employee order by salary desc limit 1,2; 从第二条开始取两条