Unterabfragen in der FROM-Klausel

Datenbearbeitung in SQL

Mona Khalil

Data Scientist, Greenhouse Software

Unterabfragen in FROM

Organisiere deine Daten neu und wandle sie um
- Daten vor der Auswahl von lang in breit umwandeln
- Daten vorfiltern
Berechne Aggregate von Aggregaten
- Welche drei Mannschaften haben im Schnitt die meisten Heimtore geschossen?
  1. Für jede Mannschaft AVG berechnen
  2. Drei höchsten AVG-Werte abrufen

Unterabfragen mit 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 |

Hauptabfragen mit FROM


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)

Hauptabfragen mit FROM


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

Hauptabfragen mit FROM

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

Hauptabfragen mit FROM

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   |

Merkenswertes

Du kannst mehrere Unterabfragen in einer FROM-Klausel erstellen.
- Weise ihnen einen Aliasnamen zu!
- Joine sie!
Du kannst eine Unterabfrage mit einer Tabelle in FROM zusammenführen.
- Füge in beiden Tabellen eine Spalte für die Zusammenführung ein!

Lass uns üben!

Datenbearbeitung in SQL