mysql学习笔记(四):常见函数之分组函数

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
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

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 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;