SQL 行转列+窗口函数的实例

今天继续和大家分享 HackerRank 上的 SQL 编程挑战的解题思路，这一次的题目叫做“Occupations”，属于中等难度级别，答案提交的成功率在 90% 左右。

问题描述

有一个职业表（OCCUPATIONS ），它有两个字段：Name、Occupation。其中，Occupation 的值只有四种：Doctor、Professor、Singer 和 Actor 。

OCCUPATIONS 表的表结构及数据如下：

Column	Type
Name	String
Occupation	String

OCCUPATIONS 的表结构

Name	Occupation
Samantha	Doctor
Julia	Actor
Maria	Actor
Meera	Singer
Ashley	Professor
Ketty	Professor
Christeen	Professor
Jane	Actor
Jenny	Doctor
Priya	Singer

OCCUPATIONS 的表数据

要求：

将数据按照 Doctor、Professor、Singer 、Actor 的顺序分成四列输出；
每一列的数据从上到下按姓名的字母顺序升序排序；
人员不足的职位列上用 NULL 填充。

输出结果：

Jenny    Ashley     Meera  Jane
Samantha Christeen  Priya  Julia
NULL     Ketty      NULL   Maria

解决方案

一般遇到“将一列中的所有唯一值作为新的列名输出”这种需求，我们首先想到的就应该是行转列，我之前写过一篇介绍SQL 行转列的通用实现的文章，感兴趣的朋友点进来看看。

做行转列时分组的依据是什么呢？即依据哪个字段分组。答案是依据每个职位中姓名的排序序号作为分组条件，而每个职位里面姓名的出现的序号可通过窗口函数求得。

先来看看对职业为 “Actor” 和 “Doctor” 的数据做组内排序。

SELECT
  occupation as p,
  row_number () over (
    PARTITION BY occupation
ORDER BY name
) AS rn,
name
FROM
  OCCUPATIONS
WHERE
  occupation IN ('Actor','Doctor')

排序的结果：

p      rn name
------ -- --------
Actor  1  Eve
Actor  2  Jennifer
Actor  3  Ketty
Actor  4  Samantha
Doctor 1  Aamina
Doctor 2  Julia
Doctor 3  Priya

最后，我们把窗口函数和行转列结合起来，就可以实现题目所要求的输出。

WITH t AS
(SELECT
  occupation,
  row_number () over (
    PARTITION BY occupation
ORDER BY NAME
) AS rn,
NAME
FROM
  OCCUPATIONS)
SELECT
  MAX(
    IF(occupation = 'Doctor', NAME, NULL)
  ) AS Doctor,
  MAX(
    IF(
      occupation = 'Professor',
      NAME,
      NULL
    )
  ) AS Professor,
  MAX(
    IF(occupation = 'Singer', NAME, NULL)
  ) AS Singer,
  MAX(IF(occupation = 'Actor', NAME, NULL)) AS Actor
FROM
  t
GROUP BY rn

如果你的 MySQL 数据库的版本在 8.0 之前，那么可以用用户变量替代窗口函数实现组内排序的功能。

SELECT
  MAX(
    IF(occupation = 'Doctor', NAME, NULL)
  ) AS Doctor,
  MAX(
    IF(
      occupation = 'Professor',
      NAME,
      NULL
    )
  ) AS Professor,
  MAX(
    IF(occupation = 'Singer', NAME, NULL)
  ) AS Singer,
  MAX(IF(occupation = 'Actor', NAME, NULL)) AS Actor
FROM
  (SELECT
    NAME,
    occupation,
    @rn := IF(@oc = occupation, @rn + 1, 1) AS seq,
    @oc := occupation
  FROM
    occupations a,
    (SELECT
      @rn := 0,
      @oc := NULL) b
  ORDER BY occupation,name
  ) t
  GROUP BY seq