Subconsultas na instrução FROM

Manipulação de dados em SQL

Mona Khalil

Data Scientist, Greenhouse Software

Subconsultas no FROM

Reestruture e transforme seus dados
- Transformar os dados de longos para largos antes de selecionar
- Pré-filtrar dados
Calcular agregados de agregados
- Quais são os 3 times com a maior média de gols marcados em casa?
  1. Calcule a média (AVG) de cada time
  2. Pegue os 3 maiores valores de AVG

Subconsultas FROM...

SELECT
  t.team_long_name AS team,
  AVG(m.home_goal) AS home_avg
FROM match AS m
LEFT JOIN team AS t
ON m.hometeam_id = t.team_api_id
WHERE season = '2011/2012'
GROUP BY team;

| team                 | home_avg         |
|----------------------|------------------|
| 1. FC Köln           | 1.13725490196078 |
| 1. FC Nürnberg       | 1.27058823529412 |
| 1. FSV Mainz 05      | 1.43697478991597 |
| AC Ajaccio           | 1.12280701754386 |

...para as consultas principais!


FROM (SELECT
          t.team_long_name AS team,
          AVG(m.home_goal) AS home_avg
      FROM match AS m
      LEFT JOIN team AS t
      ON m.hometeam_id = t.team_api_id
      WHERE season = '2011/2012'
      GROUP BY team)

...para as consultas principais!


FROM (SELECT
          t.team_long_name AS team,
          AVG(m.home_goal) AS home_avg
      FROM match AS m
      LEFT JOIN team AS t
      ON m.hometeam_id = t.team_api_id
      WHERE season = '2011/2012'
      GROUP BY team) AS subquery

...para as consultas principais!

SELECT team, home_avg
FROM (SELECT
          t.team_long_name AS team,
          AVG(m.home_goal) AS home_avg
      FROM match AS m
      LEFT JOIN team AS t
      ON m.hometeam_id = t.team_api_id
      WHERE season = '2011/2012'
      GROUP BY team) AS subquery

...para as consultas principais!

SELECT team, home_avg
FROM (SELECT
          t.team_long_name AS team,
          AVG(m.home_goal) AS home_avg
      FROM match AS m
      LEFT JOIN team AS t
      ON m.hometeam_id = t.team_api_id
      WHERE season = '2011/2012'
      GROUP BY team) AS subquery
ORDER BY home_avg DESC
LIMIT 3;

| team           | home_avg |
|----------------|----------|
| FC Barcelona   | 3.8421   |
| Real Madrid CF | 3.6842   |
| PSV            | 3.3529   |

Coisas para lembrar

Você pode criar várias subconsultas em uma instrução FROM
- Crie um alias!
- Faça um join!
Você pode juntar uma subconsulta a uma tabela no FROM
- Inclua colunas de junção nas duas tabelas!

Vamos praticar!

Manipulação de dados em SQL