Subconsultas no SELECT

Manipulação de dados em SQL

Mona Khalil

Data Scientist, Greenhouse Software

SELECT do quê?

Retorna um único valor
- Inclua valores agregados para comparar com valores individuais

Usado em cálculos matemáticos
- Desvio da média

Subconsultas no SELECT

Calcular o total de partidas em todas as temporadas

SELECT COUNT(id) FROM match;

Subconsultas no SELECT

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         |

Subconsultas no SELECT

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         |

Subconsultas SELECT para cálculos matemáticos

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';

Subconsultas no SELECT

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 |

Subconsultas SELECT: coisas para se ter em mente

Precisa retornar um ÚNICO valor
- Caso contrário, gera um erro
Certifique-se de que todos os filtros estão nos lugares certos
- Filtre corretamente a consulta principal E a subconsulta!

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 praticar!

Manipulação de dados em SQL