查询

selelct 

where   

group by

having

order by

limit

--having 前面必须得有group by

--order by 按照字段名或者统计结果排序

 

 

分组函数

 sum,count,avg,max ,min

#select sum(salary) as '总工资' from employee

#select count(id) from employee --如果count(字段名)，那么会排除该字段是NULL的值
#select count(1) from employee
#select count(*) from employee

#select sum(salary) from employee where month=1

分组统计

#select month,avg(salary) from employee group by month

where和having的区别

（1）where后面是不允许使用分组函数，having后面可以加分组函数

（2）where是用于"原表"中记录的筛选，having是用于统计结果的筛选

 

#select month,avg(salary) from employee  group by month having avg(salary)>1500
#select month,avg(salary) as agv_salary from employee  group by month having agv_salary>1500