MySQL進階4：常見函數（分組（統計）函數）

# 二、分組函數

/*
功能：用於統計使用，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、以上函數都忽略null值
3、可以和distinct搭配實現去重的運算
4、count的詳細
一般使用count(*)統計行數
5、和分組函數一同
*/

# 1. 簡單使用
SELECT SUM(salary) FROM employees;
SELECT AVG(salary) FROM employees;
SELECT MAX(salary) FROM employees;
SELECT MIN(salary) FROM employees;
SELECT COUNT(salary) FROM employees;

SELECT 
	SUM(salary) AS 總數,
	ROUND(AVG(salary),2) AS 平均,
	MAX(salary) AS 最大,
	MIN(salary) AS 最小,
	COUNT(salary) AS 個數
FROM
	employees;
	
# 2.參數支持哪些類型
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.忽略null
SELECT SUM(commission_pct),AVG(commission_pct) FROM employees;
SELECT MAX(commission_pct),MIN(commission_pct) FROM employees;

# 4.和distinct搭配
SELECT SUM(DISTINCT salary), SUM(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;