【MySQL数据库基础：4.分组函数】
目录
分组函数
用作统计使用，又称为聚合函数或统计函数或组函数
分类：
sum 求和、avg 平均值、max 最大值、min 最小值、count 计算个数
特点：
1.sum，avg-一般用于处理数值型
max，min，count可以处理任何类型
2.以上分组函数都忽略null
3.可以和distinct搭配实现去重的运算
4.count函数的单独介绍
1.简单的使用
1.1：SELECT SUM(salary) FROM employees;
SELECT AVG(salary) FROM employees;
SELECT MIN(salary) FROM employees;
SELECT MAX(salary) FROM employees;
SELECT COUNT(salary) FROM employees;
SELECT SUM(salary) FROM employees;
SELECT AVG(salary) FROM employees;
SELECT MIN(salary) FROM employees;
SELECT MAX(salary) FROM employees;
SELECT COUNT(salary) FROM employees;查询结果如下：
1.2：SELECT SUM(salary) 和,AVG(salary) 平均,MAX(salary) 最高,MIN(salary) 最低,COUNT(salary)个数
FROM employees;
SELECT SUM(salary) 和,AVG(salary) 平均,MAX(salary) 最高,MIN(salary) 最低,COUNT(salary)个数
FROM employees;查询结果如下：
2.参数支持哪些类型
2.1：SELECT SUM(last_name) ,AVG(last_name) FROM employees;
SELECT SUM(hiredate) ,AVG(hiredate) FROM employees;
SELECT MAX(last_name),MIN(last_name) FROM employees;
SELECT MAX(hiredate),MIN(hiredate)FROM employees;
3.是否忽略null
SELECT SUM(comission_put),AVG(comission_put),SUM(comission_put)/35,SUM(comission_put)/107 FROM employees;
SELECT SUM(comission_put) ,AVG(comission_put) ,
SUM(comission_put)/35,SUM(comission_put)/107
FROM employees;查询结果如下：
4.和distinct（去重）搭配
SELECT SUM(DISABLE salary),SUM(salary) FROM employees;
SELECT COUNT(DISABLE salary),COUNT(salary) FROM employees;
SELECT * FROM employees;
SELECT SUM(DISABLE salary),SUM(salary) FROM employees;查询结果如下：
5.count函数的介绍
SELECT COUNT(salary) FROM employees;
SELECT COUNT(*) FROM employees;
SELECT COUNT(salary) FROM employees;
SELECT COUNT(*) FROM employees;查询结果如下：
效率：
myisam存储引擎下，count(*)的效率高
innodb存储引擎下，count(*）和count（1）的效率差不多，比count（字段）要高一些
6.和分组函数统一查询的字段有限制
SELECT AVG(salary),employees_id FROM employees;
SELECT * FROM employees;
SELECT AVG(salary),employees_id FROM employees;查询结果如下：
分组函数所有代码：
SELECT * FROM employees;
SELECT SUM(salary) FROM employees;
SELECT AVG(salary) FROM employees;
SELECT MIN(salary) FROM employees;
SELECT MAX(salary) FROM employees;
SELECT COUNT(salary) FROM employees;
SELECT SUM(salary) 和,AVG(salary) 平均,MAX(salary) 最高,MIN(salary) 最低,COUNT(salary)个数
FROM employees;
SELECT SUM(salary) 和,ROUND(AVG(salary),2) 平均,MAX(salary) 最高,MIN(salary) 最低,COUNT(salary)个数
FROM employees;
SELECT SUM(last_name) ,AVG(last_name) FROM employees;
SELECT SUM(hiredate) ,AVG(hiredate) FROM employees;
SELECT MAX(last_name),MIN(last_name) FROM employees;
SELECT MAX(hiredate),MIN(hiredate)FROM employees;
SELECT SUM(comission_put) ,AVG(comission_put) ,SUM(comission_put)/35,SUM(comission_put)/107 FROM employees;
SELECT * FROM employees;
SELECT SUM(DISABLE salary),SUM(salary) FROM employees;
SELECT COUNT(DISABLE salary),COUNT(salary) FROM employees;
SELECT COUNT(salary) FROM employees;
SELECT COUNT(*) FROM employees;
SELECT * FROM employees;
SELECT AVG(salary),employees_id FROM employees;如果对您有帮助，点个关注吧，持续更新中，谢谢支持。如果有问题或错误，欢迎指出与我联系，谢谢。
