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+## 笔记
+
+timestampdiff（）：是mysql中用来计算两个日期或时间之间的差值的函数，该函数返回两个日期/时间之间的差值，可以指定差值的单位（秒、分钟、小时、天等）
+
+```mysql
+语法：
+timestampdiff( unit,start_date,end_date);
+-- unit : 表示差值的单位，可以是一下值之一：microsecond（微秒），second（秒），minute（分）。。。
+-- start_date : 表示时间段的起始时间
+-- end_date : 表示时间段的结束时间
+```
+
+
+
+## 作业
+
+```mysql
+create database db_emp charset utf8;
+use db_emp;
+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 部门编号,count(*) over (partition by dname order by dname) '员工总数' from employee ;
+#求每个部门的平均工资
+ select distinct dname 部门编号,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 desc) as dname,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 (partition by dname order by birth) as ename,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 ROW_NUMBER() over (partition by dname order by hiredate) as ename,hiredate from employee;
+#计算每个员工的工资与该部门平均工资的差额
+ SELECT eid, ename, dname, salary, AVG(salary) OVER (PARTITION BY dname) - salary AS 差额 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,
+       CASE
+           WHEN LAG(salary) OVER (ORDER BY salary) > salary THEN LAG(salary) OVER (ORDER BY salary)
+           ELSE salary
+       END AS 比较高的工资
+FROM employee
+ORDER BY salary;
+#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, 
+       (LAG(salary) OVER (ORDER BY salary) + salary + LEAD(salary) OVER (ORDER BY salary)) / 3 AS 平均工资
+FROM employee
+ORDER BY salary;
+#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 DESC ROWS BETWEEN CURRENT ROW AND UNBOUNDED FOLLOWING) AS 工资总和
+FROM employee
+ORDER BY salary;
+#计算每个部门内最高薪资与平均薪资的差额
+SELECT dname, MAX(salary) - AVG(salary) AS 薪资差额
+FROM employee
+GROUP BY dname;
+#找出各部门年薪第二高的员工
+SELECT dname, ename, salary
+FROM (
+    SELECT dname, ename, salary,
+           ROW_NUMBER() OVER (PARTITION BY dname ORDER BY salary DESC) AS rn
+    FROM employee
+) tmp
+WHERE rn = 2;
+#查询各部门中小于等于当前员工实际薪资的比例
+SELECT
+e.dname,
+COUNT() / (SELECT COUNT() FROM employee WHERE dname = e.dname) AS ratio
+FROM
+employee e
+WHERE
+e.salary <= (SELECT salary FROM employee WHERE ename = ‘傅嘉熙’)
+GROUP BY
+e.dname;
+#查询每个员工工资在全部员工中的排名比例
+SELECT
+e.ename,
+COUNT() / (SELECT COUNT() FROM employee) AS ratio
+FROM
+employee e
+WHERE
+e.salary <= (SELECT salary FROM employee WHERE ename = ‘傅嘉熙’)
+GROUP BY
+e.ename;
+
+#查询每个部门工资排名在前25%的员工记录数
+SELECT
+e.dname,
+COUNT() AS count
+FROM
+employee e
+WHERE
+(SELECT COUNT() FROM employee WHERE dname = e.dname AND salary <= e.salary) <= (SELECT COUNT(*) FROM employee WHERE dname = e.dname) * 0.25
+GROUP BY
+e.dname;
+
+#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
+e.dname,
+e.ename,
+e.birth,
+ABS(e.birth - (SELECT birth FROM employee WHERE dname = e.dname AND birth < e.birth ORDER BY birth DESC LIMIT 1)) AS age_difference
+FROM
+employee e
+ORDER BY
+e.dname,
+e.birth;
+#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
+e.ename,
+e.hiredate,
+DATEDIFF((SELECT hiredate FROM employee WHERE hiredate > e.hiredate ORDER BY hiredate ASC LIMIT 1), e.hiredate) AS day_difference
+FROM
+employee e
+ORDER BY
+e.hiredate;
+#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
+e.dname,
+CASE WHEN e.salary <= (SELECT AVG(salary) FROM employee WHERE dname = e.dname) THEN ‘低工资’ ELSE ‘高工资’ END AS salary_group
+FROM
+employee e
+ORDER BY
+e.dname;
+#将所有员工按照工龄分为4个组，并统计每个组的人数
+SELECT
+CASE
+WHEN YEAR(CURDATE()) - YEAR(e.hiredate) < 5 THEN ‘0-4年’
+WHEN YEAR(CURDATE()) - YEAR(e.hiredate) < 10 THEN ‘5-9年’
+WHEN YEAR(CURDATE()) - YEAR(e.hiredate) < 15 THEN ‘10-14年’
+ELSE ‘15年以上’
+END AS work_experience_group,
+COUNT(*) AS count
+FROM
+employee e
+GROUP BY
+work_experience_group;
+#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
+CASE
+WHEN salary <= (SELECT PERCENTILE_CONT(0.33) WITHIN GROUP (ORDER BY salary) FROM employee) THEN ‘低工资’
+WHEN salary <= (SELECT PERCENTILE_CONT(0.67) WITHIN GROUP (ORDER BY salary) FROM employee) THEN ‘中等工资’
+ELSE ‘高工资’
+END AS salary_group,
+AVG(salary) AS average_salary,
+MIN(salary) AS salary_range_start,
+MAX(salary) AS salary_range_end
+FROM
+employee
+GROUP BY
+salary_group;
+
+
+### -- 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
+
+#返回员工的实际年龄，如果小于当前日期则减1岁
+
+#求每个员工还有多少天过生日，并返回下次生日是星期几
+
+#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
+
+
+```
+