窗口分析函数19_Mysql查询窗口函数里第一个 最后一个 第N个元素的值的案例详解（FIRST_VALUE LAST_VALUE NVH_VALUE）

Mysql查询窗口函数之按序号取元素详解

需求概述

查询以课程(course)分区以分数(score)逆序的窗口里的第一个、最后一个和第2个元素对应的值。示例表数据见下：

SELECT '数学' course, 40 score, '张三' name UNION
SELECT  '数学' course,  60 score, '李四' name UNION
SELECT  '数学' course, 70 score, '王二' name UNION
SELECT  '数学' course, 55 score,  '赵钱' name UNION
SELECT  '英语' course,  65 score,  '张三' name UNION
SELECT  '英语' course,  70 score, '李四' name UNION
SELECT  '语文' course,  83 score, '张三' name

问题分析

该需求需要借助窗口函数的FIRST_VALUE、LAST_VALUE、NTH_VALUE函数来实现，其中针对窗口里的最后一个元素需要界定它的范围。

SQL代码

SELECT FIRST_VALUE(score) OVER w first_value,
LAST_VALUE(score) OVER w as last_value,

LAST_VALUE(score) OVER (
 PARTITION BY course
        ORDER BY score DESC
RANGE BETWEEN
     UNBOUNDED PRECEDING AND
            UNBOUNDED FOLLOWING) reallast_value,

NTH_VALUE(score,3) OVER w as nth_value,
score,
course
FROM
(
SELECT '数学' course, 40 score, '张三' name UNION
SELECT  '数学' course,  60 score, '李四' name UNION
SELECT  '数学' course, 70 score, '王二' name UNION
SELECT  '数学' course, 55 score,  '赵钱' name UNION
SELECT  '英语' course,  65 score,  '张三' name UNION
SELECT  '英语' course,  70 score, '李四' name UNION
SELECT  '语文' course,  83 score, '张三' name 
)A
WINDOW w AS (PARTITION BY course order by score DESC)

执行结果