From b9ba4da985c644e6c2f8a37948c5d1c360b50f8a Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?16=20=E9=98=99=E8=8B=8F=E6=96=87?= <2361635242@qq.com>
Date: Mon, 16 Oct 2023 20:49:01 +0800
Subject: [PATCH] =?UTF-8?q?=E4=BD=9C=E4=B8=9A?=
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+# 笔记
+
+## 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：
+
+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()、rank()、dense_rank()
+
+
+
+### 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)
+
+### 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) 
+
+### 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)
+
+### 其他函数
+
+nth_value(列名,n) —— 返回截止到当前行，该列的第n个值
+
+# 作业
+
+```mysql
+### -- 窗口函数
+
+#求每个部门的员工总数
+select distinct dname,count(eid) over (partition by dname) from employee;
+
+#求每个部门的平均工资
+select distinct dname,avg(salary) over (partition by dname) 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 dname,ename,salary,rank() over (order by salary desc) 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,datediff(now(),birth) div 365 ,rank() over (order by datediff(now(),birth) div 365 desc) 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,hiredate,datediff(now(),hiredate) div 365 ,rank() over (order by datediff(now(),hiredate) div 365 desc) from employee;
+
+#计算每个员工的工资与该部门平均工资的差额
+select dname,ename,(salary-avg(salary) over (partition by dname)) 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 (rows between 1 preceding and 1 following) 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 (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,sum(salary) over (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)-avg(salary) over (partition by dname) from employee;
+
+#找出各部门年薪第二高的员工
+select * from (select dname,eid,ename,salary,dense_rank() over (partition by dname order by salary desc) 排序 from employee) a where 排序=2;
+
+#查询各部门中小于等于当前员工实际薪资的比例
+select dname,ename,salary,cume_dist() over (partition by dname order by salary asc) from employee;
+
+#查询每个员工工资在全部员工中的排名比例
+select ename,salary,cume_dist() over (order by salary) from employee; from employee;
+
+#查询每个部门工资排名在前25%的员工记录数
+select * from (select *,ntile(4) over (partition by dname order by salary desc) a from employee) b where a=1;
+
+#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 dname,ename,birth,datediff(birth,lag(birth) over (partition by dname order by birth)) 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
+
+#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 dname,ename,salary,ntile(2) over (partition by dname order by salary) 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 b.a,count(b.a) from (select dname,ename,ntile(4) over (order by birth) a from employee) b group by b.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 num,avg(salary),min(salary),max(salary) from (select *,ntile(3) over (order by salary) num from employee) b group by num;
+
+### -- 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,floor(datediff(curdate(),hiredate)/365),case when floor(datediff(curdate(),hiredate)/365)<5 then '新员工' when floor(datediff(curdate(),hiredate)/365)>15 then '老员工' else '骨灰级员工' end '员工判定' 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,birth,floor(datediff(curdate(),birth)/365)-case when date_format(curdate(),'%m%d')<date_format(birth,'%m%d') then 1 else 0 end '年龄' 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,datediff(concat_ws('                            ',year(curdate()),month(birth),day(birth)),curdate()),date_format(concat_ws('-',year(curdate()),month(birth),day(birth)),'%W') 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 e.ename,e.dname,e.start_sal,e.salary,concat(format(((e.salary-e.start_sal)/e.start_sal) * 100,2),'%') from employee e;
+```
+
-- 
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