mysql-分組函數

**分組函數**
功能：用作統計使用，又稱爲聚合函數或統計函數或組函數
分類：
sum 求和、avg 平均值、max 最大值 、min 最小值 、count 計算個數
特點：
1、sum、avg一般用於處理數值型
   max、min、count可以處理任何類型
2、以上分組函數都忽略null值

3、可以和distinct搭配實現去重的運算

4、count函數的單獨介紹
一般使用count(*)用作統計行數

5、和分組函數一同查詢的字段要求是group by後的字段

#1、簡單 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) 個數
FROM employees;

SELECT SUM(salary) 和,ROUND(AVG(salary),2) 平均,MAX(salary) 最高,MIN(salary) 最低,COUNT(salary) 個數
FROM employees;

#2、參數支持哪些類型

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函數的詳細介紹

SELECT COUNT(salary) FROM employees;


SELECT COUNT(*) FROM employees;

SELECT COUNT(1) FROM employees;

效率：
MYISAM存儲引擎下  ，COUNT(*)的效率高
INNODB存儲引擎下，COUNT(*)和COUNT(1)的效率差不多，比COUNT(字段)要高一些


#6、和分組函數一同查詢的字段有限制

SELECT AVG(salary),employee_id  FROM employees;