MYSQL聚合函数

DDL

CREATE TABLE `studentid` (
  `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) unsigned 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=19 DEFAULT CHARSET=utf8;

DML

INSERT INTO studentid VALUES(0,'2023-12-21','盘古','123456','29833376362',240,'神','开天辟地第一人');
INSERT INTO studentid VALUES(0,'2023-12-21','女娲','123456','29833376362',240,'女','造娃第一人');
INSERT INTO studentid VALUES(0,'2023-12-21','伏羲','123456','29833376362',240,'神','先天八卦');
INSERT INTO studentid VALUES(0,'2023-12-21','螺祖','123456','29833376362',225,'神','教会百姓养蚕织布');
INSERT INTO studentid VALUES(0,'2023-12-21','神农','123456','29833376362',225,'神','尝百草，死于断肠草下');
INSERT INTO studentid VALUES(0,'2023-12-21','蚩尤','123456','29833376362',225,'男','锻造大神，坐下食铁巨兽');
INSERT INTO studentid(username,age,introduce)  VALUES('仓颉',124,'中国形象文字');

DQL

#5个常用的聚合函数count(x），max(x），min(x），sum（x),avg(x);
SELECT count(*) FROM `studentid` where pwd is not null;
SELECT max(age) '最大年龄',min(age)'最小年龄'from studentid;
SELECT sum(age）'总年龄' from studentid;
SELECT avg(age) '平均年龄'from studentid;