MySQL--基础六（分组函数）

旧城等待， 2021-08-13 20:03 407阅读 0赞

#### 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 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 DIFFRENCE
    FROM employees;
    
    SELECT DATEDIFF(MAX(hiredate),MIN(hiredate)) DIFFRENCE
    FROM 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;