MySQL进阶4：常见函数（分组（统计）函数）

# 二、分组函数

/*
功能：用于统计使用，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、以上函数都忽略null值
3、可以和distinct搭配实现去重的运算
4、count的详细
一般使用count(*)统计行数
5、和分组函数一同
*/

# 1. 简单使用
SELECT SUM(salary) FROM employees;
SELECT AVG(salary) FROM employees;
SELECT MAX(salary) FROM employees;
SELECT MIN(salary) FROM employees;
SELECT COUNT(salary) FROM employees;

SELECT 
	SUM(salary) AS 总数,
	ROUND(AVG(salary),2) AS 平均,
	MAX(salary) AS 最大,
	MIN(salary) AS 最小,
	COUNT(salary) AS 个数
FROM
	employees;
	
# 2.参数支持哪些类型
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.忽略null
SELECT SUM(commission_pct),AVG(commission_pct) FROM employees;
SELECT MAX(commission_pct),MIN(commission_pct) FROM employees;

# 4.和distinct搭配
SELECT SUM(DISTINCT salary), SUM(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;