分組函數:統計使用,聚合函數和統計函數或組函數;
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可以處理任何數據類型
2.以上分組函都忽略null值
3.和distinct搭配使用
4.用count(*)統計行數
/*
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可以處理任何數據類型
2.以上分組函都忽略null值
3.和distinct搭配使用
4.用count(*)統計行數
*/
#1.簡單使用
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) AS 和,AVG(salary) AS 平均值, MIN(salary) AS 最小值, MAX(salary) AS 最大值,COUNT(salary) AS 個數
FROM employees;
SELECT SUM(salary) AS 和,ROUND(AVG(salary),2) AS 平均值, MIN(salary) AS 最小值, MAX(salary) AS 最大值,COUNT(salary) AS 個數
FROM employees;
#3.和distinct搭配使用
SELECT SUM(DISTINCT salary),SUM(salary) FROM employees;
#4.count函數的詳細使用
SELECT COUNT(salary) FROM employees;
SELECT COUNT(*) FROM employees;
SELECT COUNT(1) FROM employees;
#測試題:
#(1)查詢員工工資的最大值,最小值,平均值,總和;
SELECT MAX(salary), MIN(salary), ROUND(AVG(salary),2),SUM(salary) FROM employeeS;
#(2)查詢員工表中的最大入職時間和最小入職時間的相差天數(DIFFRENCE)
SELECT DATEDIFF(MAX(hiredate) ,MIN(hiredate)) AS DIFFRENCE FROM employees;
#(3)查詢部門編號爲90的員工個數
SELECT COUNT(*) FROM employees WHERE department_id = 90;
#