Subconsultas correlacionadas

Manipulação de dados em SQL

Mona Khalil

Data Scientist, Greenhouse Software

Subconsulta correlacionada

Usa valores da consulta externa para gerar um resultado
Repete para cada linha gerada no conjunto de dados final
Usada para juntar, filtrar e avaliar dados de forma avançada

Um exemplo simples

Em quais etapas das partidas costuma haver mais gols do que a média?

SELECT 
    s.stage,
    ROUND(s.avg_goals,2) AS avg_goal,
    (SELECT AVG(home_goal + away_goal) FROM match 
     WHERE season = '2012/2013') AS overall_avg 
FROM 
    (SELECT
         stage,
         AVG(home_goal + away_goal) AS avg_goals
     FROM match
     WHERE season = '2012/2013'
     GROUP BY stage) AS s
WHERE s.avg_goals > (SELECT AVG(home_goal + away_goal) 
                     FROM match 
                     WHERE season = '2012/2013');

Um exemplo simples

Em quais etapas das partidas costuma haver mais gols do que a média?

SELECT 
    s.stage,
    ROUND(s.avg_goals,2) AS avg_goal,
    (SELECT AVG(home_goal + away_goal) 
     FROM match 
     WHERE season = '2012/2013') AS overall_avg 
FROM (SELECT
        stage,
        AVG(home_goal + away_goal) AS avg_goals
      FROM match
      WHERE season = '2012/2013'
      GROUP BY stage) AS s -- Subquery in FROM
WHERE s.avg_goals > (SELECT AVG(home_goal + away_goal) 
                     FROM match 
                     WHERE season = '2012/2013'); -- Subquery in WHERE

Um exemplo correlacionado

SELECT
    s.stage,
    ROUND(s.avg_goals,2) AS avg_goal,
    (SELECT AVG(home_goal + away_goal) 
     FROM match 
     WHERE season = '2012/2013') AS overall_avg 
FROM 
    (SELECT
         stage,
         AVG(home_goal + away_goal) AS avg_goals
     FROM match
     WHERE season = '2012/2013'
     GROUP BY stage) AS s
WHERE s.avg_goals > (SELECT AVG(home_goal + away_goal) 
                     FROM match AS m 
                     WHERE s.stage > m.stage);

Um exemplo correlacionado

| stage | avg_goals |
|-------|-----------|
| 3     | 2.83      |
| 4     | 2.8       |
| 6     | 2.78      |
| 8     | 3.09      |
| 10    | 2.96      |

Subconsultas simples vs. correlacionadas

Subconsulta simples

Pode ser executada independentemente da consulta principal
Avaliada uma vez em toda a consulta

Subconsulta correlacionada

Dependente da consulta principal a ser executada
Avaliada em loops
- Aumenta bastante o tempo de execução

Subconsultas correlacionadas

Qual é a média de gols marcados em cada país?

SELECT
  c.name AS country,
  AVG(m.home_goal + m.away_goal) 
     AS avg_goals
FROM country AS c
LEFT JOIN match AS m
ON c.id = m.country_id
GROUP BY country;

| country     | avg_goals        |
|-------------|------------------|
| Belgium     | 2.89344262295082 |
| England     | 2.76776315789474 |
| France      | 2.51052631578947 |
| Germany     | 2.94607843137255 |
| Italy       | 2.63150867823765 |
| Netherlands | 3.14624183006536 |
| Poland      | 2.49375          |
| Portugal    | 2.63255360623782 |
| Scotland    | 2.74122807017544 |
| Spain       | 2.78223684210526 |
| Switzerland | 2.81054131054131 |

Subconsultas correlacionadas

Qual é a média de gols marcados em cada país?

SELECT
  c.name AS country,
  (SELECT 
     AVG(home_goal + away_goal)
   FROM match AS m
   WHERE m.country_id = c.id) 
     AS avg_goals
FROM country AS c
GROUP BY country;

| country     | avg_goals        |
|-------------|------------------|
| Belgium     | 2.89344262295082 |
| England     | 2.76776315789474 |
| France      | 2.51052631578947 |
| Germany     | 2.94607843137255 |
| Italy       | 2.63150867823765 |
| Netherlands | 3.14624183006536 |
| Poland      | 2.49375          |
| Portugal    | 2.63255360623782 |
| Scotland    | 2.74122807017544 |
| Spain       | 2.78223684210526 |
| Switzerland | 2.81054131054131 |

Vamos praticar!

Manipulação de dados em SQL