SELECT의 서브쿼리

SQL에서의 데이터 조작

Mona Khalil

Data Scientist, Greenhouse Software

무엇을 SELECT하나요?

단일 값 반환
- 개별 값과 비교할 집계값 포함

수학 계산에 사용
- 평균 대비 편차 계산

SELECT의 서브쿼리

모든 시즌의 총 경기 수 계산

SELECT COUNT(id) FROM match;

SELECT의 서브쿼리

SELECT
  season,
  COUNT(id) AS matches,
  12837 as total_matches
FROM match
GROUP BY season;

| season    | matches | total_matches |
|-----------|---------|---------------|
| 2011/2012 | 3220    | 12837         |
| 2012/2013 | 3260    | 12837         |
| 2013/2014 | 3032    | 12837         |
| 2014/2015 | 3325    | 12837         |

SELECT의 서브쿼리

SELECT
  season,
  COUNT(id) AS matches,
  (SELECT COUNT(id) FROM match) as total_matches
FROM match
GROUP BY season;

| season    | matches | total_matches |
|-----------|---------|---------------|
| 2011/2012 | 3220    | 12837         |
| 2012/2013 | 3260    | 12837         |
| 2013/2014 | 3032    | 12837         |
| 2014/2015 | 3325    | 12837         |

수학 계산용 SELECT 서브쿼리

SELECT AVG(home_goal + away_goal) 
FROM match
WHERE season = '2011/2012';

2.72

SELECT
  date,
  (home_goal + away_goal) AS goals,
  (home_goal + away_goal) - 2.72 AS diff
FROM match
WHERE season = '2011/2012';

SELECT의 서브쿼리

SELECT
  date,
  (home_goal + away_goal) AS goals,
  (home_goal + away_goal) - 
     (SELECT AVG(home_goal + away_goal) 
      FROM match
      WHERE season = '2011/2012') AS diff
FROM match
WHERE season = '2011/2012';

| date       | goals | diff              |
|------------|-------|-------------------|
| 2011-07-29 | 3     | 0.28354037267081  |
| 2011-07-30 | 2     | -0.71645962732919 |
| 2011-07-30 | 4     | 1.28354037267081  |
| 2011-07-30 | 1     | -1.71645962732919 |

SELECT 서브쿼리 — 유의사항

반드시 단일 값을 반환해야 함
- 그렇지 않으면 에러 발생
필터가 올바른 위치에 있는지 확인
- 메인 쿼리와 서브쿼리 모두 적절히 필터링!

SELECT
    date,
    (home_goal + away_goal) AS goals,
    (home_goal + away_goal) - 
       (SELECT AVG(home_goal + away_goal) 
        FROM match
        WHERE season = '2011/2012') AS diff
FROM match
WHERE season = '2011/2012';

¡Vamos a practicar!

SQL에서의 데이터 조작