/*

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

max、min、count可以处理任何类型

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

*/

举例：

#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：

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、和分组函数一同查询的字段有限制

SELECT AVG(salary),employee_id  FROM employees;