MYSQL五个常见的聚合函数

学生表DDL

CREATE TABLE `student` (
  `id` int(11) NOT NULL AUTO_INCREMENT COMMENT '学号',
  `createDate` datetime DEFAULT NULL,
  `userName` varchar(20) DEFAULT NULL,
  `pwd` varchar(36) DEFAULT NULL,
  `phone` varchar(11) DEFAULT NULL,
  `age` tinyint(3) unsigned DEFAULT NULL,
  `sex` char(2) DEFAULT '男',
  `introduce` varchar(255) DEFAULT NULL,
  PRIMARY KEY (`id`)
) ENGINE=InnoDB AUTO_INCREMENT=1 DEFAULT CHARSET=utf8;

插入学生信息DML

insert into student values(0,'2024-02-25 10:10:10','赵灵儿','123',
'15612345678',16,'女','逍遥哥哥，你终于找到我了。');
insert into student values(0,'2024-02-25 10:10:10','王语嫣','123',
'15612345678',17,'女','慕容复，我和你不共戴天。');
insert into student values(0,'2024-02-25 10:10:10','龙姑娘','123',
'15612345678',22,'女','我想过过过儿过过的日子。');
insert into student values(0,'2024-02-25 10:10:10','杨过','123',
'15612345678',18,'男','一遇杨过误终身。');
insert into student values(0,'2024-02-25 10:10:10','杨逍','123',
'15612345678',27,'男','杨过跟程英的大儿子。');
insert into student (userName,age,introduce)values('黄衣女子',26,'杨过与龙姑娘的大女儿。');

五个聚合函数

# 常用的5个聚合函数 count（X）计数器，max(x)最大值MIn(x)最小值，avg(x)平均数，sum(x)求和
select count(*)from student where pwd is not null;
#max与min
select max(age) '最大年龄',min(age) '最小年龄' from student;
#平均年龄
select avg(age) '平均年龄' from student;
#年龄总和
select sum(age) '年龄总和' from student;
#利用sum计算平均数
select avg(age) '平均年龄',sum(age)/(select count(*)from student)'sum的平均年龄' from student;