分组函数

#二、分组函数
/*
功能：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

2、以上分组函数都忽略null值


3、可以和distinct搭配实现去重的运算


4、count函数的单独介绍
一般使用count(*)用作统计行数


5、和分组函数一同查询的字段要求是group by后的字段


*/




#1、简单 的使用
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;


#2、参数支持哪些类型


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 COUNT(commission_pct) FROM employees;
SELECT COUNT(last_name) FROM employees;


#3、是否忽略null


SELECT SUM(commission_pct) ,AVG(commission_pct),SUM(commission_pct)/35,SUM(commission_pct)/107 FROM employees;


SELECT MAX(commission_pct) ,MIN(commission_pct) FROM employees;


SELECT COUNT(commission_pct) FROM employees;
SELECT commission_pct FROM employees;




#4、和distinct搭配


SELECT SUM(DISTINCT salary),SUM(salary) FROM employees;


SELECT COUNT(DISTINCT salary),COUNT(salary) FROM employees;






#5、count函数的详细介绍


SELECT COUNT(salary) FROM employees;




SELECT COUNT(*) FROM employees;


SELECT COUNT(1) FROM employees;


效率：

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、和分组函数一同查询的字段有限制

文章来源: 分组函数