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+# 窗口函数
+
+窗口函数的语法结构是：
+
+```mysql
+函数 OVER（[PARTITION BY 字段名 ORDER BY 字段名 ASC|DESC]）
+```
+
+或者是：
+
+```mysql
+函数 OVER 窗口名 … WINDOW 窗口名 AS （[PARTITION BY 字段名 ORDER BY 字段名 ASC|DESC]）
+```
+
+* OVER 关键字指定函数窗口的范围。
+  * 如果省略后面括号中的内容，则窗口会包含满足WHERE条件的所有记录，窗口函数会基于所有满足WHERE条件的记录进行计算。
+  * 如果OVER关键字后面的括号不为空，则可以使用如下语法设置窗口。
+* 窗口名：为窗口设置一个别名，用来标识窗口。
+* PARTITION BY子句：指定窗口函数按照哪些字段进行分组。分组后，窗口函数可以在每个分组中分别执行。
+* ORDER BY子句：指定窗口函数按照哪些字段进行排序。执行排序操作使窗口函数按照排序后的数据记录的顺序进行编号。
+* FRAME子句：为分区中的某个子集定义规则，可以用来作为滑动窗口使用。
+
+### 序号函数
+
+row_number()、rank()、dense_rank()
+
+
+
+### 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
+```
+公用表表达式
+公用表表达式（或通用表表达式）简称为CTE（Common Table Expressions）。CTE是一个命名的临时结 果集，作用范围是当前语句。CTE可以理解成一个可以复用的子查询，当然跟子查询还是有点区别的， CTE可以引用其他CTE，但子查询不能引用其他子查询。所以，可以考虑代替子查询。
+
+依据语法结构和执行方式的不同，公用表表达式分为 普通公用表表达式 和 递归公用表表达式 2 种。
+1.普通公用表表达式
+ITH CTE名称
+AS （子查询）
+SELECT|DELETE|UPDATE 语句;
+
+with 
+   a as (  select department_id, min(hire_date) a1 from employees GROUP BY department_id	),
+   b as (  select department_id, max(hire_date) b1 from employees GROUP BY department_id	)
+select a.department_id,a1 最早入职,b1 最晚入职 from a,b where a.department_id = b.department_id;
+
+普通公用表表达式类似于子查询，不过，跟子查询不同的是，它可以被多次引用，而且可以被其他的普 通公用表表达式所引用。
+
+2.递归公用表表达式
+递归公用表表达式也是一种公用表表达式，只不过，除了普通公用表表达式的特点以外，它还有自己的特点，就是可以调用自己。它的语法结构是：
+
+```mysql
+WITH RECURSIVE
+CTE名称 AS （子查询）
+SELECT|DELETE|UPDATE 语句;
+```
+
+递归公用表表达式由 2 部分组成，分别是种子查询和递归查询，中间通过关键字 UNION [ALL]进行连接。 这里的种子查询，意思就是获得递归的初始值。这个查询只会运行一次，以创建初始数据集，之后递归 查询会一直执行，直到没有任何新的查询数据产生，递归返回。
+
+
+
+
+
+
+
+# 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
+SELECT *,salary+(year(now())-year(hiredate))*50 实发薪资 from employee;
+
+
+#求每个部门的员工总数
+SELECT DISTINCT dname,COUNT(*) over (PARTITION by dname) 员工总数 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 *,RANK() over (PARTITION by dname ORDER BY salary) 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 *,RANK() over (ORDER BY birth) 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 *,RANK() over (ORDER BY hiredate) FROM employee;
+#计算每个员工的工资与该部门平均工资的差额
+WITH
+a as (SELECT eid,avg(salary) over (PARTITION by dname) avgsal FROM employee),
+b as (SELECT * from employee)
+SELECT *,salary-avgsal 平均工资的差额 from b,a WHERE a.eid=b.eid;
+
+#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 *,ROW_NUMBER() over (ORDER BY 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 *,ROW_NUMBER() over (ORDER BY 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 *,ROW_NUMBER() over (ORDER BY salary) ,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
+
+with
+a as(SELECT DISTINCT dname,max(salary) over (PARTITION by dname) msal FROM employee),
+b as(SELECT * from employee),
+c as(SELECT DISTINCT a.dname,max(salary) over (PARTITION by dname) msa from b,a WHERE a.dname=b.dname and b.salary != a.msal)
+SELECT b.* from b,c WHERE b.dname=c.dname and salary=msa;
+
+#查询各部门中小于等于当前员工实际薪资的比例
+select ename,cume_dist() over (PARTITION by dname ORDER BY salary asc) as a from employee;
+#查询每个员工工资在全部员工中的排名比例
+select ename,CUME_DIST() over (ORDER BY salary asc) as a from employee;
+#查询每个部门工资排名在前25%的员工记录数
+with
+b as(select *,CUME_DIST() over(PARTITION by dname ORDER BY salary asc) a from employee)
+select b.dname,count(*) from employee,b where b.ename=employee.ename and a<=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 *,(year(birth)-LAG(year(birth)) over (PARTITION by dname ORDER BY birth)) as a 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 *,DATEDIFF(hiredate,lead(hiredate) over (ORDER BY hiredate)) as a 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 *,case
+	when ROW_NUMBER() over (PARTITION by dname ORDER BY salary) <=COUNT(*) over (PARTITION by dname)/2 then '低工资'
+else '高工资'	
+end as salary_group
+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 m.a,count(m.a)  from (SELECT dname, ename, NTILE(4) OVER (ORDER BY birth) a
+FROM employee)m GROUP BY m.a ;
+
+SELECT 
+    *,
+    NTILE(4) OVER (ORDER BY TIMESTAMPDIFF(YEAR, hiredate, CURDATE())) AS group_number,
+    COUNT(*) OVER (PARTITION BY NTILE(4) ORDER BY NTILE(4)) AS group_count
+FROM employee;
+#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 
+    group_number,
+    AVG(salary) AS average_salary,
+    MIN(salary) AS f,
+    MAX(salary) AS l
+FROM (
+    SELECT 
+        *,
+        NTILE(3) OVER (ORDER BY salary) AS group_number
+    FROM employee
+) AS grouped_employee
+GROUP BY group_number;
+
+### -- 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 AS actual_age
+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 AS actual_age
+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(YEAR(CURDATE()), '-', MONTH(birth), '-', DAY(birth)), CURDATE()) AS 天,
+    DATE_FORMAT(CONCAT(YEAR(CURDATE()), '-', MONTH(`birth`), '-', DAY(`birth`)), '%W') AS 星期几
+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 
+    employee.ename,
+    employee.dname,
+    employee.start_sal AS entry_salary,
+    employee.salary AS current_salary,
+    CONCAT(FORMAT(((employee.salary - employee.start_sal) / employee.start_sal) * 100, 2), '%') AS growth_rate
+FROM
+    employee;
+```
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