모든 곳에 있는 서브쿼리와 모범 사례!

SQL 데이터 조작

Mona Khalil

Data Scientist, Greenhouse Software

원하는 만큼의 서브쿼리를...

SELECT, FROM, WHERE에 여러 서브쿼리를 포함 가능

SELECT 
    s.stage,
    ROUND(s.avg_goals,2) AS avg_goal,
    (SELECT AVG(home_goal + away_goal)
     FROM match WHERE season = '2013/2014') AS overall_avg
FROM 
    (SELECT
         stage,
         AVG(home_goal + away_goal) AS avg_goals
     FROM match
     WHERE season = '2013/2014'
     GROUP BY stage) AS s
WHERE 
    s.avg_goals > (SELECT AVG(home_goal + away_goal) 
                   FROM match WHERE season = '2013/2014');

쿼리 형식 지정하기

SELECT, FROM, WHERE, GROUP BY를 정렬하기

SELECT
    col1,
    col2,
    col3
FROM table1
WHERE col1 = 2;

쿼리에 주석 달기

/* This query filters for col1 = 2
and only selects data from table1 */
SELECT
    col1,
    col2,
    col3
FROM table1
WHERE col1 = 2;

쿼리에 주석 달기

SELECT
    col1,
    col2,
    col3
FROM table1 -- this table has 10,000 rows
WHERE col1 = 2; -- Filter WHERE value 2

쿼리 들여쓰기

서브쿼리 들여쓰기하기

SELECT
    col1,
    col2,
    col3
FROM table1
WHERE col1 IN
        (SELECT id
         FROM table2
         WHERE year = 1991);

쿼리 들여쓰기

SELECT 
  date, 
  hometeam_id, 
  awayteam_id,
  CASE WHEN hometeam_id = 8455 AND home_goal > away_goal 
            THEN 'Chelsea home win'
       WHEN awayteam_id = 8455 AND home_goal < away_goal
            THEN 'Chelsea away win'
       WHEN hometeam_id = 8455 AND home_goal < away_goal
            THEN 'Chelsea home loss'
       WHEN awayteam_id = 8455 AND home_goal > away_goal
            THEN 'Chelsea away loss'
       WHEN (hometeam_id = 8455 OR awayteam_id = 8455) 
            AND home_goal = away_goal THEN 'Chelsea Tie'
       END AS outcome
FROM match
WHERE hometeam_id = 8455 OR awayteam_id = 8455;

Holywell의 SQL 스타일 가이드

그 서브쿼리가 정말 필요한가요?

서브쿼리는 연산 능력이 필요
- 데이터베이스 크기는?
- 쿼리하는 테이블의 크기는?
서브쿼리가 _꼭_ 필요할까?

서브쿼리 올바르게 필터링하기!

필터를 주의하세요!

SELECT 
    s.stage,
    ROUND(s.avg_goals,2) AS avg_goal,
    (SELECT AVG(home_goal + away_goal)
     FROM match WHERE season = '2013/2014') AS overall_avg
FROM 
    (SELECT
         stage,
         AVG(home_goal + away_goal) AS avg_goals
     FROM match
     WHERE season = '2013/2014'
     GROUP BY stage) AS s
WHERE 
    s.avg_goals > (SELECT AVG(home_goal + away_goal) 
                   FROM match WHERE season = '2013/2014');

연습해봅시다!

SQL 데이터 조작