mysql分组函数、组函数、聚合函数、统计函数

mysql分组函数、组函数、聚合函数、统计函数

直接po代码，代码中有注释

#2、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
特色：
一、sum、avg通常用于处理数值型
   max、min、count能够处理任何类型
二、以上分组函数都忽略null值
三、能够和distinct搭配实现去重的运算

四、count函数的单独介绍
通常使用count(*)用做统计行数

五、和分组函数一同查询的字段要求是group by后的字段

*/

#

#一、分组函数的简单 的使用
SELECT SUM(salary) FROM employees;
SELECT AVG(salary) FROM employees;
SELECT MAX(salary) FROM employees;
SELECT MIN(salary) FROM employees;
SELECT COUNT(salary), COUNT(*), COUNT('aaa'), COUNT("bbbb"), COUNT(666) FROM employees;
SELECT SUM(salary), AVG(salary), MAX(salary), MIN(salary), COUNT(salary)FROM employees;

#函数嵌套
SELECT SUM(salary), ROUND(AVG(salary), 2), MAX(salary), MIN(salary), COUNT(salary)FROM employees;

#二、参数支持哪些类型
#使用SUM()函数和AVG()函数去处理字符型和日期型，尽管语法不会报错，可是没什么意义
SELECT SUM(last_name), AVG(last_name) FROM employees; 
SELECT SUM(hiredate), AVG(hiredate) FROM employees;
#max、min、count能够处理任何类型
SELECT MAX(last_name), MIN(last_name) FROM employees;
SELECT MAX(hiredate), MIN(hiredate) FROM employees;
SELECT MAX(salary), MIN(salary) FROM employees;
SELECT COUNT(salary), COUNT(last_name), COUNT(*), COUNT(hiredate) FROM employees;
#COUNT()函数不会把null算进去，即忽略null
SELECT COUNT(salary), COUNT(commission_pct), COUNT(last_name), COUNT(*), COUNT(hiredate) FROM employees;

SELECT * FROM employees WHERE commission_pct IS NOT NULL;
SELECT * FROM employees WHERE commission_pct IS NULL;
#三、是否忽略null (记住：分组函数都忽略null值，即不会把null算进去)
SELECT 
  COUNT(commission_pct),
  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;
#

#四、和distinct搭配使用
SELECT COUNT(DISTINCT salary), COUNT(salary) FROM employees;
SELECT SUM(DISTINCT salary), SUM(salary) FROM employees;
SELECT SUM(DISTINCT commission_pct), SUM(commission_pct) FROM employees;
SELECT COUNT(DISTINCT commission_pct), COUNT(commission_pct) FROM employees;
#
SELECT 
  MAX(DISTINCT commission_pct) AS 结果,
  MAX(commission_pct) AS 结果,
  MIN(commission_pct) AS 结果,
  MIN(DISTINCT commission_pct) AS 结果,
  AVG(commission_pct) AS 结果,
  AVG(DISTINCT commission_pct) AS 结果,
  SUM(DISTINCT commission_pct) / COUNT(DISTINCT commission_pct) AS 结果
FROM
  employees ;
#
SELECT 
  MAX(DISTINCT salary),
  MAX(salary),
  MIN(salary),
  MIN(DISTINCT salary),
  AVG(salary),
  AVG(DISTINCT salary), 
  SUM(DISTINCT salary) / COUNT(DISTINCT salary) AS 结果
FROM
  employees ;
#别名能够取同样的，语法不会报错，可是不建议取同样的别名，容易混淆
SELECT 
  salary AS a,
  hiredate AS a 
FROM
  employees 
  /*
下面这条sql语句，语法报错，缘由是，取了同样的别名，在使用别名排序时，搞不清该别名到
底指的是谁(即模棱两可;含糊的，不明确的，引发歧义的，有两种或多种意思的)
*/
  #select salary as a,hiredate as a from employees order by a;
  
  #五、count函数的详细介绍
  
  #统计个数，推荐使用COUNT(*)，不推荐使用COUNT(字段名)
  SELECT 
    COUNT(salary),
    COUNT(commission_pct),
    #统计个数最好用*星号，不要用具体的某个字段，由于某字段中可能有null
    COUNT(*) AS 个数, 
    COUNT(NULL),
    COUNT('aaa'),
    COUNT(6666),
    COUNT("bbb") 
  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)的效率差很少，比COUNT(字段)要高一些
*/    
#
SELECT COUNT(salary), COUNT(commission_pct) FROM employees;
#
SELECT COUNT(*) FROM employees;
#
SELECT COUNT(1), COUNT(66), COUNT("bbb"), COUNT('aaa') FROM employees;

#
SELECT * FROM employees; 

#六、和分组函数一同查询的字段有限制   
#以下这条sql语句语法不会报错，可是没有任何意义
SELECT AVG(salary), employee_id, last_name FROM employees;

#测试下ROUND()函数
SELECT 
  ROUND(689.257, 2),
  ROUND('689.257', 2),
  ROUND('hello', 2),
  ROUND(
    '江西省赣州市于都县',
    4
  ) ;

#1.查询公司员工工资的最大值，最小值，平均值，总和
SELECT 
  MAX(salary),
  MIN(salary),
  AVG(salary),
  ROUND(AVG(salary), 2),
  SUM(salary) 
FROM
  employees ;
  
#DATEDIFF()函数
SELECT DATEDIFF('2018-12-18', '2018-12-01'), DATEDIFF('2018-12-18', '2018-12-2');  
SELECT DATEDIFF('18-3-8', '2018-3-01'), DATEDIFF('2018-2-12', '18-2-02');
SELECT DATEDIFF('2018-3-01', '18-3-8'), DATEDIFF('18-2-02', '2018-2-12');
#算一下本身活了多少天了
SELECT DATEDIFF(NOW(), '1995-08-27'); 

#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),使用DATEDIFF()函数
SELECT 
  MIN(hiredate) 最先入职,
  MAX(hiredate) 最晚入职,
  DATEDIFF(MAX(hiredate), MIN(hiredate)) AS 相差天数
FROM
  employees ;
  
#查看下employees表结构，查看下employees表中的hiredate字段的类型
DESC employees;

#也可使用以下方式
SELECT 
  MIN(hiredate) 最先入职,
  MAX(hiredate) 最晚入职,
  (UNIX_TIMESTAMP(MAX(hiredate)) - UNIX_TIMESTAMP(MIN(hiredate)))/(60*60*24) 相差天数,
  (UNIX_TIMESTAMP(MAX(hiredate)) - UNIX_TIMESTAMP(MIN(hiredate)))/(60*60*24*365) 相差年 
FROM
  employees ;

#3.查询部门编号为90的员工个数
#
SELECT 
  COUNT(*) 
FROM
  employees 
WHERE department_id = 90 ;

#用单引号把90包起来，效果同样
SELECT 
  COUNT(*) 
FROM
  employees 
WHERE department_id = '90' ;

#用双引号把90包起来，效果同样
SELECT 
  COUNT(*) 
FROM
  employees 
WHERE department_id = "90" ;