MySQL中的聚合函数

Posted 2020-10-10 怪咖在骚动

创建student表
CREATE TABLE IF NOT EXISTS `student` (
  `id` int(4) unsigned NOT NULL AUTO_INCREMENT,
  `name` varchar(12) NOT NULL,
  `sex` enum(‘男‘,‘女‘) NOT NULL DEFAULT ‘男‘,
  `age` tinyint(4) NOT NULL DEFAULT ‘24‘,
  `edu` enum(‘初中‘,‘高中‘,‘大专‘,‘大本‘,‘研究生‘) NOT NULL DEFAULT ‘大专‘,
  `salary` float(8,2) unsigned NOT NULL DEFAULT ‘0.00‘,
  `bonus` float(6,2) unsigned NOT NULL DEFAULT ‘0.00‘,
  `city` varchar(32) DEFAULT NULL,
  PRIMARY KEY (`id`)
) ENGINE=MyISAM  DEFAULT CHARSET=utf8 AUTO_INCREMENT=55 ;

　　

插入测试数据
INSERT INTO `student` (`id`, `name`, `sex`, `age`, `edu`, `salary`, `bonus`, `city`) VALUES
(1, ‘洪荒少女‘, ‘女‘, 24, ‘大本‘, 5000.00, 1200.00, ‘山东省‘),
(2, ‘苍老师‘, ‘女‘, 24, ‘大专‘, 750.00, 700.00, ‘陕西省‘),
(3, ‘TFboy‘, ‘男‘, 28, ‘大专‘, 8000.00, 800.00, ‘上海市‘),
(4, ‘邓超‘, ‘男‘, 25, ‘大本‘, 4000.00, 800.00, ‘山西省‘),
(5, ‘AnglaBaby‘, ‘女‘, 21, ‘大本‘, 6000.00, 2800.00, ‘山东省‘),
(6, ‘胡歌‘, ‘男‘, 24, ‘大本‘, 12300.00, 1580.00, ‘北京市‘),
(7, ‘宁泽涛‘, ‘男‘, 23, ‘大本‘, 2000.00, 530.00, ‘陕西省‘),
(8, ‘大白杨‘, ‘男‘, 24, ‘大本‘, 8900.00, 550.00, ‘河北省‘),
(9, ‘舒淇‘, ‘女‘, 30, ‘研究生‘, 24000.00, 8500.00, ‘黑龙江省‘),
(11, ‘曹广阳‘, ‘男‘, 34, ‘大本‘, 9500.00, 1200.00, ‘山东省‘),
(12, ‘王凯‘, ‘男‘, 29, ‘大专‘, 2000.00, 1400.00, ‘山东省‘),
(13, ‘郑爽‘, ‘男‘, 24, ‘大专‘, 2000.00, 1400.00, ‘河南省‘),
(14, ‘井柏然‘, ‘男‘, 24, ‘大专‘, 2000.00, 1400.00, ‘内蒙古‘),
(15, ‘陈晨‘, ‘男‘, 24, ‘大本‘, 2000.00, 1400.00, ‘云南省‘),
(16, ‘常江波‘, ‘男‘, 35, ‘大专‘, 14000.00, 840.00, ‘湖南省‘),
(17, ‘王张明‘, ‘男‘, 35, ‘大专‘, 14000.00, 840.00, ‘湖南省‘),
(18, ‘赵金生‘, ‘男‘, 37, ‘大本‘, 18000.00, 1440.00, ‘湖南省‘),
(19, ‘史月林‘, ‘女‘, 26, ‘大本‘, 8900.00, 1280.00, ‘甘肃省‘),
(20, ‘杜青青‘, ‘女‘, 27, ‘大专‘, 14900.00, 2800.00, ‘吉林省‘),
(21, ‘马俊鲲鹏‘, ‘男‘, 35, ‘高中‘, 23000.00, 3900.00, ‘江西省‘),
(22, ‘陆枝‘, ‘男‘, 24, ‘研究生‘, 2800.00, 1200.00, ‘山东省‘),
(23, ‘陆楠‘, ‘女‘, 38, ‘研究生‘, 7800.00, 1220.00, ‘北京市‘),
(24, ‘李聪‘, ‘女‘, 33, ‘研究生‘, 8800.00, 1320.00, ‘安徽省‘),
(25, ‘李鬼‘, ‘女‘, 39, ‘研究生‘, 18800.00, 2320.00, ‘安徽省‘),
(26, ‘谢媛嫒‘, ‘女‘, 25, ‘大本‘, 9400.00, 589.00, ‘河南省‘),
(27, ‘谢嫒仪‘, ‘女‘, 26, ‘高中‘, 7000.00, 520.00, ‘河北省‘),
(28, ‘袁国淇‘, ‘男‘, 27, ‘初中‘, 6800.00, 580.00, ‘河北省‘),
(29, ‘孙东明‘, ‘男‘, 24, ‘大专‘, 58000.00, 2500.00, ‘陕西省‘),
(30, ‘张震‘, ‘男‘, 35, ‘大专‘, 38000.00, 1700.00, ‘江西省‘),
(31, ‘韩振国‘, ‘男‘, 24, ‘大专‘, 28000.00, 2700.00, ‘山东省‘),
(32, ‘韩佳佳‘, ‘女‘, 29, ‘大专‘, 18000.00, 3000.00, ‘河北省‘),
(46, ‘洪荒少女‘, ‘女‘, 24, ‘大专‘, 5000.00, 1200.00, ‘山东省‘),
(34, ‘韩鹏飞‘, ‘男‘, 24, ‘大专‘, 3000.00, 3700.00, ‘湖南省‘),
(45, ‘洪荒少女‘, ‘男‘, 24, ‘大专‘, 5000.00, 700.00, NULL),
(37, ‘Mary‘, ‘女‘, 29, ‘大专‘, 3000.00, 700.00, ‘北京市‘),
(38, ‘Kate‘, ‘女‘, 29, ‘大专‘, 3000.00, 700.00, ‘上海市‘),
(36, ‘张‘, ‘女‘, 29, ‘大专‘, 3000.00, 800.00, ‘上海市‘),
(39, ‘王新劳‘, ‘男‘, 35, ‘大本‘, 7800.00, 956.00, ‘山西省‘),
(40, ‘姚新兵‘, ‘男‘, 39, ‘初中‘, 6700.00, 800.00, ‘河南省‘),
(41, ‘贾原‘, ‘男‘, 28, ‘大本‘, 8970.00, 845.00, ‘北京市‘),
(42, ‘贾似道‘, ‘男‘, 28, ‘大专‘, 23400.00, 2900.00, ‘北京市‘),
(43, ‘舒淇‘, ‘女‘, 30, ‘研究生‘, 24000.00, 8500.00, ‘黑龙江省‘),
(47, ‘洪荒少女‘, ‘女‘, 24, ‘大专‘, 5000.00, 1200.00, ‘山东省‘),
(48, ‘AnglaBaby‘, ‘女‘, 24, ‘研究生‘, 6000.00, 2800.00, ‘山东省‘),
(49, ‘AnglaBaby‘, ‘女‘, 24, ‘研究生‘, 6000.00, 2800.00, ‘山东省‘),
(50, ‘AnglaBaby‘, ‘女‘, 24, ‘研究生‘, 6000.00, 2800.00, ‘山东省‘),
(52, ‘尼采‘, ‘男‘, 24, ‘大专‘, 0.00, 500.00, NULL);　

1.定义：SQL中聚合函数对一组值进行计算，并返回单个的值

2.主要有：

　　avg():返回指定组中的平均值

　　count():返回指定组中项目的总数量　　

　　max():返回指定组中的数据最大值

　　min():返回指定组中的数据最小值

　　sum():返回指定组中的数据和

3.使用场景：聚合函数经常和select语句的group by子句一起使用

　　举例：查询出工资最高的学生的信息

　　　分析：首先查出工资最高的值，然后按照值最高作为条件再来查询出对应的学生信息。

　　　　第一步：select max(salary) as 最高工资 from student;

　　　　第二步：select * from student where salary = (select max(salary) as 最高工资 from student);

　　举例：查询出学生的最高工资，最低工资，平均工资，工资总和

　　　　select max(salary) 最高工资,min(salary) 最低工资,avg(salary) 平均工资,sum(salary) 工资总和 from student;

 　　举例:统计城市的总的数量值

　　　　select count(city) from student;

4.语法：select 字段列表 from 表名 [where 子句] group by  子句；

　　举例：查询出student表中男生和女生共有多少人

　　分析：先按照sex分组，男生一共多少，女生一共多少，最后在统计即可；也就是先分组在统计总数；

　　语句：select sex,count(*) 总数 from student group by sex;

可以同时对多个字段进行分组：

　　举例：查询出同一个城市的男生女生的平均工资

　　分析:需要按照city和sex同时分组，然后统计男女生的平均工资 

　　语句：select city,sex,avg(salary) from student group by city,sex;