Allgemeine Tabellenausdrücke

Datenbearbeitung in SQL

Mona Khalil

Data Scientist, Greenhouse Software

Beim Einsatz von Unterabfragen...

Abfragen werden schnell komplex!
- Man verliert leicht den Überblick

Lösung: Allgemeine Tabellenausdrücke (CTEs)!

Allgemeine Tabellenausdrücke

Allgemeine Tabellenausdrücke (CTEs)

Tabelle vor der Hauptabfrage deklariert
Namensgebung und Bezugnahme später in FROM-Klausel

CTEs einrichten

WITH cte AS (
    SELECT col1, col2
    FROM table)

SELECT
    AVG(col1) AS avg_col
FROM cte;

Unterabfrage in FROM einrichten

SELECT
  c.name AS country,
  COUNT(s.id) AS matches
FROM country AS c
INNER JOIN (
  SELECT country_id, id 
  FROM match
  WHERE (home_goal + away_goal) >= 10) AS s
ON c.id = s.country_id
GROUP BY country;

| country     | matches |
|-------------|---------|
| England     | 3       |
| Germany     | 1       |
| Netherlands | 1       |
| Spain       | 4       |

Direkt an den Anfang

(
  SELECT country_id, id 
  FROM match
  WHERE (home_goal + away_goal) >= 10
)

Direkt an den Anfang

WITH s AS (
  SELECT country_id, id 
  FROM match
  WHERE (home_goal + away_goal) >= 10
)

Abrufen eines CTEs

WITH s AS (
  SELECT country_id, id 
  FROM match
  WHERE (home_goal + away_goal) >= 10
)
SELECT
  c.name AS country,
  COUNT(s.id) AS matches
FROM country AS c
INNER JOIN s
ON c.id = s.country_id
GROUP BY country;

| country     | matches |
|-------------|---------|
| England     | 3       |
| Germany     | 1       |
| Netherlands | 1       |
| Spain       | 4       |

Abrufen aller CTEs

WITH s1 AS (
  SELECT country_id, id 
  FROM match
  WHERE (home_goal + away_goal) >= 10),
s2 AS (                              -- New subquery
  SELECT country_id, id 
  FROM match
  WHERE (home_goal + away_goal) <= 1
)
SELECT
  c.name AS country,
  COUNT(s1.id) AS high_scores,
  COUNT(s2.id) AS low_scores         -- New column
FROM country AS c
INNER JOIN s1
ON c.id = s1.country_id
INNER JOIN s2                        -- New join
ON c.id = s2.country_id
GROUP BY country;

Was spricht für einen CTE?

Wird einmal ausgeführt
- CTE wird im Speicher hinterlegt
- Verbessert die Abfrageleistung
Bessere Struktur von Abfragen
Bezugnahme auf andere CTEs
Bezugnahme auf CTE selbst (SELF JOIN)

Lass uns üben!

Datenbearbeitung in SQL