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+# 窗口函数
+
+mysql 8.0新增窗口函数（开窗函数）。
+
+非聚合窗口函数：是相对于聚合函数来说的，特性为非聚合。一次只处理一行数据。
+
+窗口聚合函数：窗口聚合函数在单元行上计算某个字段的结果时，可将窗口范围内的数据输入到聚合函数中，并且不会改变行数。
+
+
+
+### 语法
+
+```sql
+窗口函数名称(参数) over(partition by ... order by... 窗口大小)
+```
+
+partition by 分组,等价于group by
+
+order by 排序
+
+
+
+### 序号函数
+
+row_number()	：顺序排序，显示每条数据的行号，如：1 2 3
+
+rank()	：值相同的话，序号也会相同，会跳过，如：1 1 3
+
+dense_rank()	：值相同的话，序号也会相同，不会跳过，如：1 1 2
+
+
+
+### 开窗聚合函数
+
+```sql
+sum()|avg()|min()|max()|count()| over (partition by ... order by...)
+```
+
+案例:
+
+```sql
+# 获取各部门薪金总和(合计)
+select ename,hiredate,deptno,sal,
+sum(sal) over(partition by deptno) 'sum'
+from emp;
+
+# 获取各部门薪金总和(累加)
+select ename,hiredate,deptno,sal,
+sum(sal) over(partition by deptno order by sal) 'sum'
+from emp;
+```
+
+
+
+#### 窗口大小
+
+```sql
+# 开窗范围
+# 获取各部门薪金总和(范围：初始行至当前行)
+select ename,hiredate,deptno,sal,
+sum(sal) over(partition by deptno rows between unbounded preceding and current row) 'sum'
+from emp;
+
+-- rows 启用窗口大小
+-- between ... and ... 范围区间
+-- unbounded preceding 起始行
+-- current row 当前行
+```
+
+不同案例：
+
+```sql
+# 范围：前3行 + 当前行
+sum(sal) over(partition by deptno rows between 3 preceding and current row)
+
+# 范围：前3行 + 当前行 + 后1行
+sum(sal) over(partition by deptno rows between 3 preceding and 1 following) 
+-- following指当前行之后的行
+
+# 范围：当前行至最后一行
+sum(sal) over(partition by deptno rows between current row and unbounded following) 
+-- unbounded following 最终行
+```
+
+
+
+### 分布函数
+
+##### cume_dist()
+
+分组内 <= 或 >= 当前值的行数占比(包含当前值本身)
+
+```sql
+# 查询各部门员工小于等于当前薪资的比例
+
+select ename,deptno,sal,
+cume_dist() over(partition by deptno order by sal) '占比'
+from emp;
+
+-- order by 控制计算的列
+-- asc 小于等于
+-- desc 大于等于
+```
+
+
+
+### 前后函数
+
+返回当前行的前某一行的值，或者后某一行的值
+
+lag(列名，前第n行) 
+
+lead(列名，后第n行) 
+
+```sql
+select ename,deptno,sal,
+lag(sal,1) over(partition by deptno order by sal) '前1',
+lag(sal,2) over(partition by deptno order by sal) '前2'
+from emp;
+```
+
+
+
+### 头尾函数
+
+返回第一个或最后一个列的值
+
+first_value(列名)
+
+last_value(列名)
+
+```sql
+-- 查询各个部门中，第一个和最后一个入职的员工姓名
+select ename,deptno,sal,hiredate,
+first_value(ename) over(partition by deptno order by hiredate) 'first',
+last_value(ename) over(partition by deptno order by hiredate) 'last'
+from emp;
+```
+
+
+
+### 其他函数
+
+nth_value(列名,n) —— 返回截止到当前行，该列的第n个值
+
+```sql
+-- 查找出各个部门，薪资排名第2的员工姓名
+select ename,deptno,sal,
+nth_value(ename) over(partition by deptno order by sal) '第二名'
+from emp;
+
+-- 查询全公司，薪资排名第2的员工姓名
+-- 1
+select ename,deptno,sal,
+nth_value(ename,2) over(order by sal) '第二名'
+from emp;
+-- 2
+select ename,deptno,sal,
+nth_value(ename,2) over() '第二名'
+from emp order by sal;
+```
+
+ntile(n) —— 将分区中的有序数据分为n个等级，记录等级数
+
+```sql
+-- 将每个部门员工按照入职日期分成3组
+select ename,deptno,sal,
+ntile(3) over(partition by deptno order by hiredate) '组别'
+from emp;
+
+-- 找出每个部门中的老员工？？
+```
+
+#### 作业
+
+~~~~mysql
+
+create table if not exists `employee`
+(
+    `eid` int not null auto_increment comment '员工id' primary key,
+    `ename` varchar(20) not null comment '员工名称',
+    `dname` varchar(50) not null comment '部门名称',
+    `hiredate` datetime not null comment '入职日期',
+		`birth` date not null comment '生日',
+    `salary` double null comment '基本薪资',
+		`start_sal` double null comment '入职薪资'
+);
+
+insert into `employee` (`ename`, `dname`, `hiredate`,`birth`, `salary`,`start_sal`) values ('傅嘉熙', '开发部', '2002-08-20 12:00:04','1980-12-10', 9000,6500);
+insert into `employee` (`ename`, `dname`, `hiredate`, `birth`,`salary`,`start_sal`) values ('武晟睿', '开发部', '2002-06-12 13:54:12', '1984-2-5',9500,6000);
+insert into `employee` (`ename`, `dname`, `hiredate`, `birth`,`salary`,`start_sal`) values ('孙弘文', '开发部', '2003-10-16 08:27:06','1979-8-7', 9400,8000);
+insert into `employee` (`ename`, `dname`, `hiredate`,`birth`, `salary`,`start_sal`) values ('潘乐驹', '开发部', '2004-04-22 03:56:11','1980-5-12', 9500,6800);
+insert into `employee` (`ename`, `dname`, `hiredate`,`birth`, `salary`,`start_sal`) values ('潘昊焱', '人事部', '2007-02-24 03:40:02','1987-2-12', 5000,4500);
+insert into `employee` (`ename`, `dname`, `hiredate`,`birth`, `salary`,`start_sal`) values ('沈涛', '人事部', '2012-12-14 09:16:37','1993-4-30', 6000,5500);
+insert into `employee` (`ename`, `dname`, `hiredate`, `birth`,`salary`,`start_sal`) values ('江峻熙', '人事部', '2018-05-12 01:17:48','1990-6-8', 5000,3000);
+insert into `employee` (`ename`, `dname`, `hiredate`,`birth`, `salary`,`start_sal`) values ('陆远航', '人事部', '2018-04-14 03:35:57','1989-11-13', 5500,5000);
+insert into `employee` (`ename`, `dname`, `hiredate`, `birth`,`salary`,`start_sal`) values ('姜煜祺', '销售部', '2020-03-23 03:21:05','1995-1-1', 6000,5500);
+insert into `employee` (`ename`, `dname`, `hiredate`, `birth`,`salary`,`start_sal`) values ('邹明', '销售部', '2015-11-23 23:10:06','1996-2-19', 6800,6000);
+insert into `employee` (`ename`, `dname`, `hiredate`, `birth`,`salary`,`start_sal`) values ('董擎苍', '销售部', '2012-02-12 07:54:32','1985-10-7', 6500,4800);
+insert into `employee` (`ename`, `dname`, `hiredate`,`birth`, `salary`,`start_sal`) values ('钟俊驰', '销售部', '2010-04-10 12:17:06','1981-3-25', 6000,3500);
+
+
+# 员工按工龄，每年增加50元薪水。实发薪资 = 基本薪资 + 工龄 * 50
+
+
+
+
+### -- 窗口函数
+
+
+
+#求每个部门的员工总数
+select distinct dname,count(ename) over (partition by dname) 总人数 from employee;
+
+#求每个部门的平均工资
+select distinct dname,avg(salary) over (partition by dname) 平均工资 from employee;
+
+#求每个部门的工资排名（从高到低,相同工资并列，并执行跳过排序）
+select ename,dname,salary,rank() over (partition by dname order by salary desc) 工资排名 from employee;
+
+#求公司所有员工的年龄排序（相同年龄并列，执行跳过排序）
+select ename,birth,rank() over (order by birth) 年龄排名 from employee;
+
+#求每个部门的员工工龄排序（相同年龄并列，执行顺序排序）
+select ename,row_number() over (order by hiredate) 工龄排名,datediff(now(),hiredate) 工作天数 from employee;
+
+#计算每个员工的工资与该部门平均工资的差额
+select ename,dname,salary,avg(salary) over (partition by dname) 部门平均工资,salary-avg(salary) over (partition by dname) 差额 from employee;
+
+
+
+select * from employee;
+#按员工工资进行排序，比较相邻两个员工的工资，输出比较高的工资
+select ename,salary,
+	max(salary) over ( order by salary rows between current row and 1 following) 工资比较, 
+from employee; 
+#按员工工资进行排序，查询当前员工与前一位和后一位的工资平均值
+select ename,salary,
+	avg(salary) over (order by salary rows between 1 preceding and 1 following)
+from employee; 
+#按员工工资进行排序，查询当前员工至最后一位员工的工资总和
+select ename,salary,
+	sum(salary) over (order by salary rows between current row and unbounded following )
+from employee; 
+#计算每个部门内最高薪资与平均薪资的差额
+select distinct dname,
+	max(salary) over (partition by dname) 最高薪资, 
+	avg(salary) over (partition by dname) 平均薪资,
+	max(salary) over (partition by dname)-avg(salary) over (partition by dname) as 差额  
+from employee;
+
+#找出各部门年薪第二高的员工
+select a.dname,a.salary,a.排名 from 
+	(select dname,salary,dense_rank() over (partition by dname order by salary) 排名 from employee) a 
+where a.排名 =2;
+#查询各部门中 小于等于当前员工实际薪资的比例
+select ename,salary,cume_dist() over (partition by dname order by salary) as 员工实际薪资的比例 from employee; 
+
+#查询每个员工工资在全部员工中的排名比例
+select ename,salary,sum(salary) over (order by salary) 总工资,salary/sum(salary) over () as 排名比例 from employee;
+#查询每个部门工资排名在前25%的员工记录数
+select * from employee where eid in(
+	select eid from 
+	(select eid,dname,cume_dist() over (order by salary) 工资比例 from employee) as a 
+where a.工资比例 >= 0.25);
+#每个部门按年龄进行排序，求当前员工与前一位员工的年龄差
+select ename,birth,
+		datediff(lag(birth,1) over (order by birth),birth) 年龄差
+from employee;
+#按入职日期进行排序，查询公司每个员工与后面一个员工的入职天数差异
+select ename,hiredate,
+	datediff(lead(hiredate,1) over (order by hiredate),hiredate) 入职天数差异
+from employee;
+#将每个部门的员工按工资平均分为2个组，组1为低工资，组2为高工资
+select ename,dname,salary, 
+	ntile(2) over (partition by dname order by salary) 等级
+from employee;
+#将所有员工按照工龄分为4个组，并统计每个组的人数
+select *,count(工龄组) over(partition by 工龄组) 组的人数 
+	from
+(select ename,hiredate,ntile(4) over (order by hiredate) 工龄组 from employee) a;
+#将员工按照工资分为3个组，并统计组别，每组平均工资，工资范围（first_value、last_value）
+select *,avg(salary) over (partition by 组别) as 每组平均工资,
+	concat_ws(' - ',first_value(salary) over(partition by 组别),last_value(salary) over(partition by 组别)) as 每组工资范围 
+	from 
+(select ename,salary,ntile(3) over (order by salary ) as 组别 from employee) a;
+
+### -- 非窗口函数
+
+#按照工龄区分等级，小于5年为新员工,5-15年为老员工,大于15年为骨灰级员工,输出姓名，部门，工龄，级别
+select ename,dname,hiredate,(datediff(now(),hiredate)/365) as 工龄,
+	case when (datediff(now(),hiredate)/365) < 5 then '新员工'
+			 when (datediff(now(),hiredate)/365) < 15 then '老员工'  
+			 else '骨灰级员工' end as 级别 
+from employee;
+#返回员工的实际年龄，如果小于当前日期则减1岁
+select ename,dname,floor((datediff(now(),birth)/365)) as 年龄 from employee;
+
+
+select * from employee;
+#求每个员工还有多少天过生日，并返回下次生日是星期几
+select ename,
+	abs(datediff(concat_ws('-',year(curdate()),month(birth),day(birth)),curdate())) as 距离生日天数,
+	date_format(concat_ws('-',year(curdate())+1,month(birth),day(birth)),'%W') as 下次生日星期
+from employee;
+
+#求每个员工当前实发工资与入职时工资的增长率,输出员工姓名,部门,入职工资,实际工资,增长率
+select ename,dname,hiredate,salary,round((salary-start_sal) / start_sal,2) as 增长率 from employee
+
+
+~~~~
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