Subconsultas en la sentencia FROM

Manipulación de datos en SQL

Mona Khalil

Data Scientist, Greenhouse Software

Subconsultas en FROM

Reestructura y transforma tus datos
- Transformar los datos de largos a anchos antes de la selección
- Prefiltrado de datos
Cálculo de agregados de agregados
- ¿Qué 3 equipos tienen la media más alta de goles en casa marcados?
  1. Calcula AVG para cada equipo
  2. Obtén los 3 valores de AVG más altos

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 las consultas principales!


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 las consultas principales!


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 las consultas principales!

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 las consultas principales!

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   |

Cosas que debes recordar

Puedes crear varias subconsultas en una sentencia FROM
- ¡Alias para ellos!
- ¡Unión para ellos!
Puedes unir una subconsulta a una tabla en FROM
- ¡Incluye columnas de unión en ambas tablas!

¡Vamos a practicar!

Manipulación de datos en SQL