mysql-聚合函數

mysql中聚合函數其實只有5個，

1.COUNT()函數：用來統計記錄的條數；

2.SUM()函數:是求和函數；

3.AVG()函數:是求平均值的函數 ;

4.MAX()函數是求最大值的函數

5.MIN()函數是求最小值的函數

具體的實列：

/*
SQLyog 企業版 - MySQL GUI v8.14
MySQL - 5.1.49-community
*********************************************************************
*/
/*!40101 SET NAMES utf8 */;

create table `t_grade` (
`id` int ,
`stuName` varchar (60),
`course` varchar (60),
`score` int
);
insert into `t_grade` (`id`, `stuName`, `course`, `score`) values('1','張三','語文','91');
insert into `t_grade` (`id`, `stuName`, `course`, `score`) values('2','張三','數學','90');
insert into `t_grade` (`id`, `stuName`, `course`, `score`) values('3','張三','英語','87');
insert into `t_grade` (`id`, `stuName`, `course`, `score`) values('4','李四','語文','79');
insert into `t_grade` (`id`, `stuName`, `course`, `score`) values('5','李四','數學','95');
insert into `t_grade` (`id`, `stuName`, `course`, `score`) values('6','李四','英語','80');
insert into `t_grade` (`id`, `stuName`, `course`, `score`) values('7','王五','語文','77');
insert into `t_grade` (`id`, `stuName`, `course`, `score`) values('8','王五','數學','81');
insert into `t_grade` (`id`, `stuName`, `course`, `score`) values('9','王五','英語','89');

 

SELECT COUNT(*) FROM t_grade;//統計有多少條數據

SELECT COUNT(*) AS total FROM t_grade;//把統計的條數放在totle列中

SELECT stuName,COUNT(*) FROM t_grade GROUP BY stuName;//統計出每個學生學習了幾門課程

SELECT stuName,SUM(score) FROM t_grade WHERE stuName="張三";//張三的總分

SELECT stuName,SUM(score) FROM t_grade GROUP BY stuName;//統計每個學生的總分

SELECT stuName,AVG(score) FROM t_grade WHERE stuName="張三";

SELECT stuName,AVG(score) FROM t_grade GROUP BY stuName;

SELECT stuName,course,MAX(score) FROM t_grade WHERE stuName="張三";

SELECT stuName,MAX(score) FROM t_grade GROUP BY stuName;

SELECT stuName,course,MIN(score) FROM t_grade WHERE stuName="張三";

SELECT stuName,MIN(score) FROM t_grade GROUP BY stuName;