一 引言
工作中常会遇到 “查询工资最高/第二高/最低/倒数第N的员工” “销售量前10的商品” 此类排序取前N的问题。这篇文章以查询薪资第二高/倒数第二高的员工为例，给出了三种解决方法。
二 创建表
CREATE TABLE `salaries` (
`emp_no` int(11) NOT NULL,
`salary` int(11) NOT NULL,
`from_date` date NOT NULL,
`to_date` date NOT NULL,
PRIMARY KEY (`emp_no`,`from_date`));
INSERT INTO salaries VALUES(10001,60117,'1986-06-26','1987-06-26'); INSERT INTO salaries VALUES(10001,62102,'1987-06-26','1988-06-25'); INSERT INTO salaries VALUES(10001,66074,'1988-06-25','1989-06-25'); INSERT INTO salaries VALUES(10001,66596,'1989-06-25','1990-06-25'); INSERT INTO salaries VALUES(10001,66961,'1990-06-25','1991-06-25'); INSERT INTO salaries VALUES(10001,71046,'1991-06-25','1992-06-24'); INSERT INTO salaries VALUES(10001,74333,'1992-06-24','1993-06-24'); INSERT INTO salaries VALUES(10001,75286,'1993-06-24','1994-06-24'); INSERT INTO salaries VALUES(10001,75994,'1994-06-24','1995-06-24'); INSERT INTO salaries VALUES(10001,76884,'1995-06-24','1996-06-23'); INSERT INTO salaries VALUES(10001,80013,'1996-06-23','1997-06-23'); INSERT INTO salaries VALUES(10001,81025,'1997-06-23','1998-06-23'); INSERT INTO salaries VALUES(10001,81097,'1998-06-23','1999-06-23'); INSERT INTO salaries VALUES(10001,84917,'1999-06-23','2000-06-22'); INSERT INTO salaries VALUES(10001,85112,'2000-06-22','2001-06-22'); INSERT INTO salaries VALUES(10001,85097,'2001-06-22','2002-06-22'); INSERT INTO salaries VALUES(10001,88958,'2002-06-22','9999-01-01'); INSERT INTO salaries VALUES(10002,72527,'1996-08-03','1997-08-03'); INSERT INTO salaries VALUES(10002,72527,'1997-08-03','1998-08-03'); INSERT INTO salaries VALUES(10002,72527,'1998-08-03','1999-08-03'); INSERT INTO salaries VALUES(10002,72527,'1999-08-03','2000-08-02'); INSERT INTO salaries VALUES(10002,72527,'2000-08-02','2001-08-02'); INSERT INTO salaries VALUES(10002,72527,'2001-08-02','9999-01-01'); INSERT INTO salaries VALUES(10003,40006,'1995-12-03','1996-12-02'); INSERT INTO salaries VALUES(10003,43616,'1996-12-02','1997-12-02'); INSERT INTO salaries VALUES(10003,43466,'1997-12-02','1998-12-02'); INSERT INTO salaries VALUES(10003,43636,'1998-12-02','1999-12-02'); INSERT INTO salaries VALUES(10003,43478,'1999-12-02','2000-12-01'); INSERT INTO salaries VALUES(10003,43699,'2000-12-01','2001-12-01'); INSERT INTO salaries VALUES(10003,43311,'2001-12-01','9999-01-01'); INSERT INTO salaries VALUES(10004,40054,'1986-12-01','1987-12-01'); INSERT INTO salaries VALUES(10004,42283,'1987-12-01','1988-11-30'); INSERT INTO salaries VALUES(10004,42542,'1988-11-30','1989-11-30'); INSERT INTO salaries VALUES(10004,46065,'1989-11-30','1990-11-30'); INSERT INTO salaries VALUES(10004,48271,'1990-11-30','1991-11-30'); INSERT INTO salaries VALUES(10004,50594,'1991-11-30','1992-11-29'); INSERT INTO salaries VALUES(10004,52119,'1992-11-29','1993-11-29'); INSERT INTO salaries VALUES(10004,54693,'1993-11-29','1994-11-29'); INSERT INTO salaries VALUES(10004,58326,'1994-11-29','1995-11-29'); INSERT INTO salaries VALUES(10004,60770,'1995-11-29','1996-11-28'); INSERT INTO salaries VALUES(10004,62566,'1996-11-28','1997-11-28'); INSERT INTO salaries VALUES(10004,64340,'1997-11-28','1998-11-28'); INSERT INTO salaries VALUES(10004,67096,'1998-11-28','1999-11-28'); INSERT INTO salaries VALUES(10004,69722,'1999-11-28','2000-11-27'); INSERT INTO salaries VALUES(10004,70698,'2000-11-27','2001-11-27'); INSERT INTO salaries VALUES(10004,74057,'2001-11-27','9999-01-01'); INSERT INTO salaries VALUES(10005,78228,'1989-09-12','1990-09-12'); INSERT INTO salaries VALUES(10005,82621,'1990-09-12','1991-09-12'); INSERT INTO salaries VALUES(10005,83735,'1991-09-12','1992-09-11'); INSERT INTO salaries VALUES(10005,85572,'1992-09-11','1993-09-11'); INSERT INTO salaries VALUES(10005,85076,'1993-09-11','1994-09-11'); INSERT INTO salaries VALUES(10005,86050,'1994-09-11','1995-09-11'); INSERT INTO salaries VALUES(10005,88448,'1995-09-11','1996-09-10'); INSERT INTO salaries VALUES(10005,88063,'1996-09-10','1997-09-10'); INSERT INTO salaries VALUES(10005,89724,'1997-09-10','1998-09-10'); INSERT INTO salaries VALUES(10005,90392,'1998-09-10','1999-09-10'); INSERT INTO salaries VALUES(10005,90531,'1999-09-10','2000-09-09'); INSERT INTO salaries VALUES(10005,91453,'2000-09-09','2001-09-09'); INSERT INTO salaries VALUES(10005,94692,'2001-09-09','9999-01-01'); INSERT INTO salaries VALUES(10006,43311,'1990-08-05','1991-08-05'); INSERT INTO salaries VALUES(10006,43311,'1991-08-05','1992-08-04'); INSERT INTO salaries VALUES(10006,43311,'1992-08-04','1993-08-04'); INSERT INTO salaries VALUES(10006,43311,'1993-08-04','1994-08-04'); INSERT INTO salaries VALUES(10006,43311,'1994-08-04','1995-08-04'); INSERT INTO salaries VALUES(10006,43311,'1995-08-04','1996-08-03'); INSERT INTO salaries VALUES(10006,43311,'1996-08-03','1997-08-03'); INSERT INTO salaries VALUES(10006,43311,'1997-08-03','1998-08-03'); INSERT INTO salaries VALUES(10006,43311,'1998-08-03','1999-08-03'); INSERT INTO salaries VALUES(10006,43311,'1999-08-03','2000-08-02'); INSERT INTO salaries VALUES(10006,43311,'2000-08-02','2001-08-02'); INSERT INTO salaries VALUES(10006,43311,'2001-08-02','9999-01-01'); INSERT INTO salaries VALUES(10007,56724,'1989-02-10','1990-02-10'); INSERT INTO salaries VALUES(10007,60740,'1990-02-10','1991-02-10'); INSERT INTO salaries VALUES(10007,62745,'1991-02-10','1992-02-10'); INSERT INTO salaries VALUES(10007,63475,'1992-02-10','1993-02-09'); INSERT INTO salaries VALUES(10007,63208,'1993-02-09','1994-02-09'); INSERT INTO salaries VALUES(10007,64563,'1994-02-09','1995-02-09'); INSERT INTO salaries VALUES(10007,68833,'1995-02-09','1996-02-09'); INSERT INTO salaries VALUES(10007,70220,'1996-02-09','1997-02-08'); INSERT INTO salaries VALUES(10007,73362,'1997-02-08','1998-02-08'); INSERT INTO salaries VALUES(10007,75582,'1998-02-08','1999-02-08'); INSERT INTO salaries VALUES(10007,79513,'1999-02-08','2000-02-08'); INSERT INTO salaries VALUES(10007,80083,'2000-02-08','2001-02-07'); INSERT INTO salaries VALUES(10007,84456,'2001-02-07','2002-02-07'); INSERT INTO salaries VALUES(10007,88070,'2002-02-07','9999-01-01'); INSERT INTO salaries VALUES(10008,46671,'1998-03-11','1999-03-11'); INSERT INTO salaries VALUES(10008,48584,'1999-03-11','2000-03-10'); INSERT INTO salaries VALUES(10008,52668,'2000-03-10','2000-07-31'); INSERT INTO salaries VALUES(10009,60929,'1985-02-18','1986-02-18'); INSERT INTO salaries VALUES(10009,64604,'1986-02-18','1987-02-18'); INSERT INTO salaries VALUES(10009,64780,'1987-02-18','1988-02-18'); INSERT INTO salaries VALUES(10009,66302,'1988-02-18','1989-02-17'); INSERT INTO salaries VALUES(10009,69042,'1989-02-17','1990-02-17'); INSERT INTO salaries VALUES(10009,70889,'1990-02-17','1991-02-17'); INSERT INTO salaries VALUES(10009,71434,'1991-02-17','1992-02-17'); INSERT INTO salaries VALUES(10009,74612,'1992-02-17','1993-02-16'); INSERT INTO salaries VALUES(10009,76518,'1993-02-16','1994-02-16'); INSERT INTO salaries VALUES(10009,78335,'1994-02-16','1995-02-16'); INSERT INTO salaries VALUES(10009,80944,'1995-02-16','1996-02-16'); INSERT INTO salaries VALUES(10009,82507,'1996-02-16','1997-02-15'); INSERT INTO salaries VALUES(10009,85875,'1997-02-15','1998-02-15'); INSERT INTO salaries VALUES(10009,89324,'1998-02-15','1999-02-15'); INSERT INTO salaries VALUES(10009,90668,'1999-02-15','2000-02-15'); INSERT INTO salaries VALUES(10009,93507,'2000-02-15','2001-02-14'); INSERT INTO salaries VALUES(10009,94443,'2001-02-14','2002-02-14');
三 查询第三高薪资，如果没有第三高，用null表示
四 查询to_date="9999-01-01"，薪资最高的员工
1. max函数只能取出一条
2. 改进一下，把结果作为where子查询
思路：查询出最高工资，作为子查询，where salary=最高工资的员工
五 查询to_date="9999-01-01"，薪资第二高的员工
先把所有的salary查出来看一下，第二高的是88958。
1. 法一： 第二名的特殊查法
这种方法只能用于取第二名。取其他的TopN不能用，如果有多个并列第二工资的员工只能查出一个
2. 法一修改，可以查出所有工资并列第二的员工
法一的基础上嵌套一下，salary作为查询条件，法一的代码作为子查询。
思路：查询出第二工资作为子查询，where salary=第二工资的员工
3. 法二：order by + limit
这种方法实际上取出的是排列后第二行的记录，只能取出一个记录，无法查出所有并列第二的记录
如果有两个薪资并列第一，查出来的可能是薪资第一的记录。
这题第一名和第二名只有一个，可以用这种方法，但实际上并不严谨。
4. 法二修改，可以查出所有工资并列第二的员工
思路：查询出第二工资作为子查询（salary加distinct)，where salary=第二工资的员工
5 法三：排名法（通用方法）
原理：假如 a 排第10，意味着前面有9个数比a大，有10个数大于等于a， 求某个数的排名就是求前面有几个数大于等于这个数
思路：重复用salaries表，命名为s1,s2。用where语句s1.salary<=s2.salary筛选出所有大于等于s1.salary的s2.salary。然后统计每个s1.salary有多少个s2.salary大于等于它，就能得到每个s1.salary的排名
5.1 步骤一
重复用salaries表，命名为s1,s2。用where语句s1.salary<=s2.salary筛选出所有大于等于s1.salary的s2.salary。
例如：当salary=88958时，大于等于它的有88958，84692 共2个，所以salary=88958排第2
5.2 步骤二
count( distinct s2.salary) ：统计每个s1.salary有多少个s2.salary大于等于它得到每个s1.salary的排名
5.3 步骤三
那怎么取得第二名的数呢？ 只需要限制条件，rank=2
不用group by （使用group by会导致s1.salary被去重）
六 查询to_date="9999-01-01",薪资倒数第二的员工
先把所有的salary查出来看一下，倒数第二高是72527。
1. 法一：order by + limit
这种方法用在这里为什么出错了呢？ 因为 limit 1，1 取出来的是第二行的值。这题中，最高薪水有两个，取出来的第二行是第二个最高薪水。
2. 法一改进：给 salary 加 distinct 去重
去重后，第一名只有一行记录，limit 1，1取到的是第二的薪水。但这种方法只能查询salary，不能查出对应的员工信息
3. 法一再改进：salary作为查询条件，嵌套子查询
4. 法三： 排名法
上面说过，求某个数的排名就是求前面有几个数大于等于这个数，假如 a 排第10，意味着前面有9个数比a大，有10个数大于等于a。
那倒数也是一样的，假如a排倒数第二，意味着比它小的数只有一个，有两个数小于等于它（比它小的数和它本身）
求某个数的倒数排名，就是求小于等于它的数有几个
从结果可以看出，salary=72527时，只有72525本身和43311比它小。说明salary=72525可能是倒数第二的薪资。接下来，只需要统计 每个s1.salary 对应的比它小的 s2.salay 有几个，就能得到salary的倒数名次。
那么怎么用代码查询倒数第二呢，和前面查正数排名一样，只需要限制排名=2
不用group by （使用group by会导致s1.salary被去重）
六 查询to_date="9999-01-01",薪资前三名
法一：order by+limit
实际上取出的是排在前三行的工资，这题的数据集工资没有重复，可以这样做，但如果工资有重复，这样做就不严谨
法二：排名法
七 给员工薪资排名
八 其他
思路：可以看成两个表，一个是id,name,salary. managerid 另一个是managerid,name,salary。
通过 managerid关联两个表。变成 id,name,salary,managerid,managerid(1),salary.这样就把员工salary和经理salary匹配起来了。
但现在是同一个表，所以做法是同一个表用两次。本应在manager表里的managerid变成这里的id
