Mysql基础部分(6)---分组函数

ゝ一世哀愁。 2023-05-21 08:56 20阅读 0赞

Mysql基础部分内容：  
[Mysql基础部分(1)—基础操作指令与语法规范][Mysql_1]  
[Mysql基础部分(2)—基础查询][Mysql_2]  
[Mysql基础部分(3)—条件查询][Mysql_3]  
[Mysql基础部分(4)—排序查询][Mysql_4]  
[Mysql基础部分(5)—常见函数][Mysql_5]  
[Mysql基础部分(6)—分组函数][Mysql_6]  
[Mysql基础部分(7)—分组查询][Mysql_7]  
[Mysql基础部分(8)—sql99语法][Mysql_8_sql99]  
[Mysql基础部分(9)—子查询][Mysql_9]  
[Mysql基础部分(10)—分页查询][Mysql_10]  
[Mysql基础部分(11)—联合查询][Mysql_11]  
[Mysql基础部分(12)—数据的增删改][Mysql_12]  
[Mysql基础部分(13)—库和表的管理][Mysql_13]  
[Mysql基础部分(14)—常见的数据类型][Mysql_14]  
[Mysql基础部分(15)—常见约束][Mysql_15]  
[Mysql基础部分(16)—标识列][Mysql_16]  
[Mysql基础部分(17)—事务][Mysql_17]  
[Mysql基础部分(18)—视图][Mysql_18]  
[Mysql基础部分(19)—变量][Mysql_19]  
[Mysql基础部分(20)—存储过程][Mysql_20]  
[Mysql基础部分(21)—函数][Mysql_21]  
[Mysql基础部分(22)—流程控制结构][Mysql_22]

#二、分组函数
    /* 功能：用做统计使用，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数据 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，其他的都不行 */
    
    #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(last_name) FROM employees;
    SELECT COUNT(commission_pct) FROM employees;
    
    #3.是否忽略null值
    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;
    
    #用来统计所有行数:在每行中只要有一个不为null，则表示该行有值
    SELECT COUNT(*) FROM employees;
    
    #相当于在原来表中加了一个新的字段，所以count(1)也是统计所有行数
    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)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;

[Mysql_1]: https://blog.csdn.net/weixin_43900321/article/details/105664839
[Mysql_2]: https://blog.csdn.net/weixin_43900321/article/details/105729424
[Mysql_3]: https://blog.csdn.net/weixin_43900321/article/details/105729465
[Mysql_4]: https://blog.csdn.net/weixin_43900321/article/details/105729506
[Mysql_5]: https://blog.csdn.net/weixin_43900321/article/details/105729526
[Mysql_6]: https://blog.csdn.net/weixin_43900321/article/details/105729554
[Mysql_7]: https://blog.csdn.net/weixin_43900321/article/details/105729593
[Mysql_8_sql99]: https://blog.csdn.net/weixin_43900321/article/details/105810240
[Mysql_9]: https://blog.csdn.net/weixin_43900321/article/details/105849897
[Mysql_10]: https://blog.csdn.net/weixin_43900321/article/details/105869419
[Mysql_11]: https://blog.csdn.net/weixin_43900321/article/details/105978422
[Mysql_12]: https://blog.csdn.net/weixin_43900321/article/details/105978492
[Mysql_13]: https://blog.csdn.net/weixin_43900321/article/details/106000721
[Mysql_14]: https://blog.csdn.net/weixin_43900321/article/details/106000793
[Mysql_15]: https://blog.csdn.net/weixin_43900321/article/details/106057742
[Mysql_16]: https://blog.csdn.net/weixin_43900321/article/details/106057793
[Mysql_17]: https://blog.csdn.net/weixin_43900321/article/details/106156900
[Mysql_18]: https://blog.csdn.net/weixin_43900321/article/details/106156942
[Mysql_19]: https://blog.csdn.net/weixin_43900321/article/details/106233375
[Mysql_20]: https://blog.csdn.net/weixin_43900321/article/details/106233530
[Mysql_21]: https://blog.csdn.net/weixin_43900321/article/details/106233577
[Mysql_22]: https://blog.csdn.net/weixin_43900321/article/details/106253492