mysql学习随笔------常用聚合函数

	一般来说，比较常用的聚合函数就是count()总数  sum()和  avg()平均值  max()最大值  min()最小值，		
	聚合函数一般都是与group by 一起使用
		create table t_grade(
			id int(5) primary key auto_increment,
			stuName varchar(20) comment '姓名',
			course varchar(20) comment '科目',
			score int comment '分数'
		);

###count()函数
	select count(*) as total from t_grade;
	select stuName,count(*) as total from t_grade group by stuName;

###sum函数
	select stuName,sum(score) as '总分' from t_grade where stuName='张三';
	select stuName,SUM(score) as '总分' from t_grade group by stuName;
	
###avg()函数
	select stuName,AVG(score) as '平均分' from t_grade where stuName="张三";
	select stuName,Avg(score) as '平均分' from t_grade GROUP BY stuName;
	
###max函数
	select stuName,course ,max(score) as '最高分' from t_grade where stuName="张三";
	select stuName,MAX(score) as '最高分' from t_grade group by stuName;
	
####min函数
	select stuName,course ,min(score) as '最低分' from t_grade where stuName="张三";
	select stuName,min(score) as '最低分' from t_grade group by stuName;