[Hacker Rank] Occupations

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■ 문제

https://www.hackerrank.com/challenges/occupations/problem

Occupations | HackerRank

Pivot the Occupation column in OCCUPATIONS so that each Name is sorted alphabetically and displayed underneath its corresponding Occupation. The output column headers should be Doctor, Professor, Singer, and Actor, respectively.

Note: Print NULL when there are no more names corresponding to an occupation.

Input Format

The OCCUPATIONS table is described as follows:

Occupation will only contain one of the following values: Doctor, Professor, Singer or Actor.

Sample Input

Sample Output

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

Explanation

The first column is an alphabetically ordered list of Doctor names.
The second column is an alphabetically ordered list of Professor names.
The third column is an alphabetically ordered list of Singer names.
The fourth column is an alphabetically ordered list of Actor names.
The empty cell data for columns with less than the maximum number of names per occupation (in this case, the Professor and Actor columns) are filled with NULL values.

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■ 풀이

Pivot the Occupation column in OCCUPATIONS so that each Name is sorted alphabetically and displayed underneath its corresponding Occupation.

> 직업에 따라 이름 알파벳 순으로 정렬되도록 OCCUPATIONS 테이블 컬럼 피봇해라 ①

 

The output column headers should be Doctor, Professor, Singer, and Actor, respectively.

> 출력 컬럼은 직업이어야 한다. ②

 

  PIVOT FUNCTION TAGGING   

 CASE WHEN

 DECODE

 IF , 집계함수 (* IF와 집계함수로 피봇할 때 Key는 ROW_NUM()

 

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풀이1> MySQL

SELECT MIN(CASE WHEN occupation = 'Doctor' THEN name END) Doctor,
       MIN(CASE WHEN occupation = 'Professor' THEN name END) Professor,
       MIN(CASE WHEN occupation = 'Singer' THEN name END) Singer,
       MIN(CASE WHEN occupation = 'Actor' THEN name END) Actor
 FROM (SELECT *, ROW_NUMBER() OVER (PARTITION BY Occupation ORDER BY name) rnk
        FROM OCCUPATIONS) A
 GROUP BY rnk
 ORDER BY rnk;

 

 

단, SELECT절에 사용되는 집계 함수는 MIN, MAX 는 상관이 없다.

# from절 서브쿼리
SELECT *, ROW_NUMBER() OVER (PARTITION BY Occupation ORDER BY name) rnk
        FROM OCCUPATIONS;

> from절 서브쿼리를 따로 출력하면

 위와 같이 직업(Occupation)별 이름순으로 정렬된 후 정렬 번호(rnk)가 붙는다.

메인 쿼리는 이 결과에 대해서 rnk로 그룹화하는데 그룹화 하고나면 각 rnk당 직업별 인원은 1명밖에 존재하지않게 된다. 따라서 SELECT절에서 CASE WHEN으로 직업 조건을 주고 추출할 때, MIN, MAX중 어느 것을 써도 같은 결과를 추출할 수 있다.

 

풀이2> Oracle

SELECT MIN(DECODE(OCCUPATION, 'Doctor', Name))    Doctor
     , MIN(DECODE(OCCUPATION, 'Professor', Name)) Professor
     , MIN(DECODE(OCCUPATION, 'Singer', Name))    Singer
     , MIN(DECODE(OCCUPATION, 'Actor', Name))     Actor
  FROM (
        SELECT OCCUPATION, NAME
             , ROW_NUMBER() OVER(PARTITION BY OCCUPATION ORDER BY NAME) rnk
          FROM OCCUPATIONS
       ) A
 GROUP BY rnk
 ORDER BY 1, 2, 3, 4;