MySQL基础——分组函数（聚合函数，统计函数，组函数）

MySQL基础——分组函数（聚合函数，统计函数，组函数）

分组函数

功能：用于统计使用，又称为聚合函数，统计函数，组函数。

分类：

sum求和，avg求平均值，max最大值，min最小值 count 非null字段个数

特点：

1.sum，avg一般用于处理数值型数据，max，min，count可以处理任何数据类型

2.以上分组函数都忽略null值

3.可以和distinct搭配实现去重运算

4.count 函数单独介绍

5.和分组函数一同查询的字段要求是group by后的字段

简单的使用

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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) 总和,AVG(salary) 平均,MAX(salary) 最大值,MIN(salary) 最小值,COUNT(salary) 总数 FROM employees; 
SELECT SUM(salary) 总和,ROUND(AVG(salary),2) 平均,MAX(salary) 最大值,MIN(salary) 最小值,COUNT(salary) 总数 FROM employees; 

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(last_name) FROM employees;

忽略null

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

SELECT commission_pct FROM employees;

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

SELECT COUNT(commission_pct) FROM employees;

和distinct搭配使用

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

count函数详细介绍

SELECT COUNT(salary) FROM employees;

SELECT COUNT(*) FROM employees;

SELECT COUNT(1) FROM employees;

/*
效率
myisum存储引擎下 count(*)的效率高
innodb存储引擎下 count(*)和count(1)效率差不多，比count(字段)效率要高
*/

和分组函数一同查询的字段有限制

SELECT AVG(salary),employee_id FROM employees;
#虽然可以将employee_id查询出来，但没有任何意义