Common Table Expressions

Gegevens manipuleren in SQL

Mona Khalil

Data Scientist, Greenhouse Software

Bij het toevoegen van subqueries...

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Common Table Expressions

Common Table Expressions (CTE's)

Tabel gedefinieerd voor de hoofdquery
Genoemd en gerefereerd later in FROM statement

CTE's instellen

WITH cte AS (
    SELECT col1, col2
    FROM table)

SELECT
    AVG(col1) AS avg_col
FROM cte;

Neem een subquery in FROM

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       |

Plaats het aan het begin

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

Plaats het aan het begin

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

Toon de CTE

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       |

Toon alle CTE's

WITH s1 AS (
  SELECT country_id, id 
  FROM match
  WHERE (home_goal + away_goal) >= 10),
s2 AS (                              -- Nieuwe 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         -- Nieuwe kolom
FROM country AS c
INNER JOIN s1
ON c.id = s1.country_id
INNER JOIN s2                        -- Nieuwe join
ON c.id = s2.country_id
GROUP BY country;

Waarom CTE's gebruiken?

Eenmalig uitgevoerd
- CTE wordt dan in geheugen opgeslagen
- Verbetert query-prestaties
Betere organisatie van queries
Andere CTE's refereren
Zichzelf refereren (SELF JOIN)

Laten we oefenen!

Gegevens manipuleren in SQL