MYSQL聚合函数的使用方法

前言

数据库名称可以为【schoolDB】，字符集【utf8】，排序规则【utf8_general_ci】。

1.建表语句——DDL

​
CREATE TABLE `student` (
  `id` int(11) NOT NULL AUTO_INCREMENT COMMENT '学号',
  `createDate` datetime DEFAULT NULL COMMENT '创建时间',
  `userName` varchar(20) DEFAULT NULL COMMENT '用户名',
  `pwd` varchar(36) DEFAULT NULL COMMENT '密码',
  `phone` varchar(11) DEFAULT NULL COMMENT '手机号',
  `age` tinyint(3) DEFAULT NULL COMMENT '年龄',
  `sex` char(2) DEFAULT NULL COMMENT '性别',
  `introduce` varchar(255) DEFAULT NULL COMMENT '简介',
  PRIMARY KEY (`id`)
) ENGINE=InnoDB AUTO_INCREMENT=42 DEFAULT CHARSET=utf8;

​

2.插入语句——DML

INSERT INTO student VALUES(0,'2024-02-25 10:00:00','刘德华','123654','15612345678',62,'男','永远的男神，爱你一万年，爱你经得起考研。');
INSERT INTO student VALUES(0,'2024-02-25 10:00:00','刘青云','123654','15612345678',65,'男','真正的演员，好角色用心演绎。');
INSERT INTO student VALUES(0,'2024-02-25 10:00:00','周星驰','123654','15612345678',61,'男','所有的电影都没有对爱情有过轻视。');
INSERT INTO student VALUES(0,'2024-02-25 10:00:00','张翰','123654','15612345678',32,'男','一起坐死在流星雨下。');
INSERT INTO student VALUES(0,'2024-02-25 10:00:00','王祖贤','123654','15612345678',27,'女','你再美你能比过王祖贤吗？');
INSERT INTO student (userName,age,introduce) VALUES ('刘亦菲',37,'神仙姐姐');
SELECT * FROM student

3.查询语句——DQL

​SELECT count(*) FROM student WHERE pwd is not null;
SELECT max(age) '最大年龄',min(age) '最小年龄' FROM student;
SELECT avg(age) '平均年龄' FROM student;
SELECT avg(age) '平均年龄',sum(age)/(SELECT count(*) FROM student) 'sum计算平均年龄' FROM student;