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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

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 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#SUM：忽略  AVG：忽略 MAX：忽略 MIN：忽略 COUNT：忽略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;

#1.查询公司员工工资的最大值，最小值，平均值，总和SELECT MAX(salary) 最大值,MIN(salary) 最小值,AVG(salary) 平均值,SUM(salary) 和FROM employees;#2.查询员工表中的最大入职时间和最小入职时间的相差天数 （DIFFRENCE）SELECT MAX(hiredate) 最大,MIN(hiredate) 最小,(MAX(hiredate)-MIN(hiredate))/1000/3600/24 DIFFRENCEFROM employees;SELECT DATEDIFF(MAX(hiredate),MIN(hiredate)) DIFFRENCEFROM employees;SELECT DATEDIFF('1995-2-7','1995-2-6');#3.查询部门编号为90的员工个数SELECT COUNT(*) FROM employees WHERE department_id = 90;

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