Подзапросы везде! И рекомендации по использованию!

Манипуляции с данными в SQL

Mona Khalil

Data Scientist, Greenhouse Software

Столько подзапросов, сколько нужно...

Можно использовать несколько подзапросов в SELECT, FROM, WHERE

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');

Форматируйте запросы

Выравнивайте SELECT, FROM, WHERE и GROUP BY

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

Комментируйте запросы

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

Комментируйте запросы

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

Используйте отступы в запросах

Используйте отступы для подзапросов!

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

Используйте отступы в запросах

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;

Holywell's SQL Style Guide

Нужен ли этот подзапрос?

Подзапросы требуют вычислительных ресурсов
- Насколько велика ваша база данных?
- Насколько велика таблица, к которой вы обращаетесь?
Действительно ли подзапрос необходим?

Правильно фильтруйте каждый подзапрос!

Следите за фильтрами!

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');

Давайте потренируемся!

Манипуляции с данными в SQL