MySql基础查询-分组函数

#分组函数
#1. 查询公司员工工资的最大值，最小值，平均值，总和
SELECT SUM(salary),MAX(salary),MIN(salary),AVG(salary),COUNT(salary) FROM employees
#工资去重

SELECT    sum( DISTINCT salary ),    sum( salary ) FROM    employees

#查询有几种工资
SELECT count(DISTINCT salary) FROM employees
#统计工资的个数
SELECT count(salary) FROM employees
#统计总行数即总记录条数

SELECT count(*) FROM employeesSELECT count(1) FROM employees

#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）

SELECT    datediff(        max( hiredate ),    min( hiredate )) AS '相差天数' FROM    employees

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

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