1 #二、分组函数
  2 /*
  3 功能：用作统计使用，又称为聚合函数或统计函数或组函数
  4 
  5 分类：
  6 sum求和、avg平均值、 max最大值、min最小值、count计算个数
  7 
  8 特点：
  9 1、sum、avg一般用于处理数值型
 10    max、min、count可以处理任何类型
 11 2、以上分组函数都忽略null值
 12 3、可以和distinct搭配使用，实现去重的运算
 13 4、count函数的单独介绍
 14 一般使用count(*)用作统计行数
 15 5、和分组函数一同查询的字段要求是group by后的字段
 16 */
 17 
 18 #1.简单的使用
 19 SELECT SUM(salary) FROM employees;
 20 SELECT AVG(salary) FROM employees;
 21 SELECT MIN(salary) FROM employees;
 22 SELECT MAX(salary) FROM employees;
 23 SELECT COUNT(salary) FROM employees;
 24 
 25 SELECT SUM(salary) 和,AVG(salary) 平均,MAX(salary) 最高,
 26 MIN(salary) 最低, COUNT(salary) 个数
 27 FROM employees;
 28 
 29 SELECT SUM(salary) 和,ROUND(AVG(salary)) 平均,MAX(salary) 最高,
 30 MIN(salary) 最低, COUNT(salary) 个数
 31 FROM employees;
 32 
 33 #2.参数支持哪些类型
 34 SELECT MAX(last_name),MIN(last_name) FROM employees;
 35 
 36 SELECT MAX(hiredate),MIN(hiredate) FROM employees;
 37 
 38 SELECT COUNT(commission_pct) FROM employees;
 39 SELECT COUNT(last_name) FROM employees;
 40 
 41 #3.是否忽略null
 42 SELECT SUM(commission_pct),AVG(commission_pct),
 43 SUM(commission_pct)/35,SUM(commission_pct)/107
 44 FROM employees;
 45 
 46 SELECT MAX(commission_pct),MIN(commission_pct) FROM employees;
 47 
 48 SELECT COUNT(commission_pct) FROM employees;
 49 
 50 #4.和distinct搭配
 51 SELECT SUM(DISTINCT salary),SUM(salary) FROM employees;
 52 
 53 SELECT COUNT(DISTINCT salary),COUNT(salary) FROM employees;
 54 
 55 #5.count函数的详细介绍
 56 SELECT COUNT(salary) FROM employees;
 57 
 58 SELECT COUNT(*) FROM employees; -- 107
 59 
 60 SELECT COUNT(1) FROM employees; -- 107
 61 #效率：
 62 /*
 63 MYISAM存储引擎下，count(*)的效率最高
 64 INNODB存储引擎下，count(*)和count(1)的效率差不多，比count(字段)要高一些
 65 */
 66 
 67 #6.和分组函数一同查询的字段有限制
 68 -- select avg(salary),employee_id from employees;
 69 
 70