Common Table Expressions

Manipulation de données dans Snowflake

Jake Roach

Field Data Engineer

Common Table Expressions

Les Common Table Expressions (CTE) stockent temporairement le résultat d'une requête pour être réutilisé par une autre requête

Les CTE sont définies au début d'une requête
WITH <cte-name> AS ( <query> )
Semblables aux sous-requêtes

-- Indiquez le nom de la CTE,
-- puis la requête entre parenthèses
WITH <cte-name> AS (
    <query>
)


SELECT
    ...
FROM <cte-name>
... ;

Rapport sur les étudiant·e·s à risque

WITH at_risk AS (
    SELECT
        student_id
        course_name,
        teacher_name,
        grade
    FROM student_courses
    WHERE grade < 70 AND is_required
)


SELECT
    students.student_name,
    at_risk.*
FROM at_risk
JOIN students ON at_risk.student_id = students.id;

Résultats temporaires stockés dans at_risk
Sélection d'un sous-ensemble d'enregistrements
Interroger at_risk pour générer un rapport

Rapport sur les étudiant·e·s à risque

Affichage des résultats d'une CTE : sous-ensemble et rapport final

Comparer sous-requêtes et CTE

SELECT
    month_num,
    AVG(differential) AS avg_differential
    MIN(differential) AS most_differential
FROM (

    SELECT
        month_num,
        windchill - temperature AS differential
    FROM weather
    WHERE 
        season = 'Winter' AND
        temperature < 32

)
GROUP BY month_num;

WITH daily_temperature_differential AS (
    SELECT
        month_num,
        windchill - temperature AS differential
    FROM weather
    WHERE 
        season = 'Winter' AND 
        temperature < 32
)

SELECT
    month_num,
    AVG(differential) AS avg_differential
    MIN(differential) AS most_differential
FROM daily_temperature_differential
GROUP BY month_num;

Trouver l'écart de température

WITH daily_temperature_differential AS (
    SELECT
        month_num,
        windchill - temperature AS differential
    FROM weather
    WHERE 
        season = 'Winter' AND 
        temperature < 32
)


SELECT
    month_num,
    AVG(differential) AS avg_differential
    MIN(differential) AS most_differential
FROM daily_temperature_differential
GROUP BY month_num;

          | month_num | differential |
          | --------- | ------------ |
          |     12    |      -12     |
          |     1     |      -3      |
          |     1     |       0      |
          |     2     |      -7      |

La progression est plus naturelle qu'avec des sous-requêtes.

| month_num | avg_differential | most_differential |
| --------- | ---------------- | ----------------- |
|     12    |       -5.77      |        -14        |
|     1     |       -1.91      |        -8         |
|     2     |       -8.13      |        -22        |

Passons à la pratique !

Manipulation de données dans Snowflake