Sous-requêtes et expressions de table courantes

Introduction à Snowflake SQL

George Boorman

Senior Curriculum Manager, DataCamp

Sous-requête

Requêtes imbriquées
Utilisé dans les clauses FROM, WHERE, HAVING ou SELECT

Exemple :

SELECT column1 
FROM table1 
WHERE column1 = (SELECT column2 FROM table2 WHERE condition)

Types : Sous-requêtes corrélées et non corrélées

Sous-requête non corrélée

-- Main query returns pizzas priced at the maximum value found in the subquery
SELECT pizza_id
FROM pizzas
-- Uncorrelated subquery that identifies the highest pizza price
WHERE price = (
    SELECT MAX(price)
    FROM pizzas
)

La sous-requête n'interagit pas avec la requête principale

Sous-requête corrélée

La sous-requête fait référence aux colonnes de la requête principale.

SELECT pt.name, 
       pz.price, 
       pt.category
FROM pizzas AS pz
JOIN pizza_type AS pt 
    ON pz.pizza_type_id = pt.pizza_type_id
WHERE pz.price = (
  -- Identifies max price for each pizza category
    SELECT MAX(p2.price) -- Max price
    FROM pizzas AS p2
    WHERE -- Correlated: uses outer query column 
      p2.pizza_type_id = pz.pizza_type_id
)

Expressions de table courantes

Syntaxe générale :

-- WITH keyword
WITH cte1 AS ( -- CTE name
        SELECT col_1, col_2
            FROM table1
    )
    ...
SELECT ... 
FROM cte1 -- Query CTE
;

Expressions de table courantes

WITH max_price AS ( -- CTE called max_price
    SELECT pizza_type_id, 
           MAX(price) AS max_price
    FROM pizzas
    GROUP BY pizza_type_id
)

-- Main query 
SELECT pt.name, 
       pz.price, 
       pt.category
FROM pizzas AS pz
JOIN pizza_type AS pt ON pz.pizza_type_id = pt.pizza_type_id
JOIN max_price AS mp  -- Joining with CTE max_price
     ON pt.pizza_type_id = mp.pizza_type_id 
WHERE pz.price < mp.max_price -- Compare the price with max_price CTE column

CTE multiples

-- Define multiple CTEs separated by commas
WITH cte1 AS (
    SELECT ...
    FROM ...
),

cte2 AS (
    SELECT ...
    FROM ...
)

-- Main query combining both CTEs
SELECT ...
FROM cte1
JOIN cte2 ON ...
WHERE ...

Pourquoi utiliser les CTE ?

Gestion d'opérations complexes
Modulaire
Lisible
Réutilisable

Passons à la pratique !

Introduction à Snowflake SQL