• MySql 分组函数


    #二、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

    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;

  • 相关阅读:
    快乐
    好好思考
    中秋
    要把收拾房间养成一个习惯_611
    退出微博, 不要再寻求心里安慰。
    Proj CMI Paper Reading: Mining Input Grammars from Dynamic Taints
    Proj CMI Paper Reading: Locating Faults with Program Slicing: An Empirical Analysis
    Proj CMI Paper Reading: Mining Android Apps for Anomalies
    git操作(工作中实际开发流程)
    linux c 常用代码
  • 原文地址:https://www.cnblogs.com/cb1186512739/p/12655103.html
Copyright © 2020-2023  润新知