mysql的窗口函数，以leetcode的查找第n高的薪水为例

题目描述

sql执行顺序

CREATE FUNCTION getNthHighestSalary(N INT) RETURNS INT
BEGIN
set N=N-1;
RETURN (
# Write your MySQL query statement below.
select IFNULL((select distinct Salary from Employee order by Salary desc limit N,1),NULL) as getNthHighestSalary
);
END

CREATE FUNCTION getNthHighestSalary(N INT) RETURNS INT
BEGIN
RETURN (
# Write your MySQL query statement below.
select distinct Salary from(Select Salary,DENSE_RANK() over(order by Salary desc) num from Employee) t
where N=t.num
);
END

下面讲 mysql的窗口函数 希望动手实践

建表sql
create table tb_score(id int primary key auto_increment,stu_no varchar(10),course varchar(50),score decimal(4,1) ,key idx_stuNo_course(stu_no,course));
insert into tb_score(stu_no,course,score)values(‘2020001’,‘mysql’,90),(‘2020001’,‘C++’,85),(‘2020003’,‘English’,100),(‘2020002’,‘mysql’,50),(‘2020002’,‘C++’,70),(‘2020002’,‘English’,99);
insert into tb_score(stu_no,course,score)values(‘2020003’,‘mysql’,78),(‘2020003’,‘C++’,81),(‘2020003’,‘English’,80),(‘2020004’,‘mysql’,80),(‘2020004’,‘C++’,60),(‘2020004’,‘English’,100);
insert into tb_score(stu_no,course,score)values(‘2020005’,‘mysql’,98),(‘2020005’,‘C++’,96),(‘2020005’,‘English’,70),(‘2020006’,‘mysql’,60),(‘2020006’,‘C++’,90),(‘2020006’,‘English’,70);
insert into tb_score(stu_no,course,score)values(‘2020007’,‘mysql’,50),(‘2020007’,‘C++’,66),(‘2020007’,‘English’,76),(‘2020008’,‘mysql’,90),(‘2020008’,‘C++’,69),(‘2020008’,‘English’,86);
insert into tb_score(stu_no,course,score)values(‘2020009’,‘mysql’,70),(‘2020009’,‘C++’,66),(‘2020009’,‘English’,86),(‘2020010’,‘mysql’,75),(‘2020010’,‘C++’,76),(‘2020010’,‘English’,81);

insert into tb_score(stu_no,course,score)values(‘2020011’,‘mysql’,90),(‘2020012’,‘C++’,85)(‘2020011’,‘English’,84),(‘2020012’,‘English’,75),(‘2020013’,‘C++’,96),(‘2020013’,‘English’,88);

建好之后：

函数举例

ROW_NUMBER 分数相同时按照学号顺序进行排名
DENSE_RANK 分数相同时排名也相同
RANK 果2个并列的第1之后，下一个我想是第3名
NTILE 对每个分组排名后，再将对应分组分成N个小组

总结