Più common table expression

Tipi di dati e funzioni in Snowflake

Jacob Roach

Field Data Engineer

Common table expressions (CTE)

WITH seniors AS ( 

    SELECT 
        student_id, 
        first_name 
    FROM STUDENTS.personal_info 
    WHERE graduation_year = 2025

)


SELECT
    ...
FROM seniors

Le CTE archiviano temporaneamente i risultati di una query per usarli poi in un’altra query

Servono a organizzare le query
Più leggibili e modulari
Non limitate a una sola CTE

Definire più CTE

Più risultati temporanei si definiscono con un singolo WITH

...), <another-cte-name> AS (...)
Rende più chiaro filtrare e manipolare le tabelle
Fai JOIN di più risultati temporanei
Sottoquery dentro un’altra CTE

 WITH <cte-name> AS (

     <query>

 ), <another-cte-name> AS (

     -- Aggiungi un’altra query!
     <another-query>
 )


 -- Queste CTE si possono unire con JOIN 
 SELECT ... ;

Unire risultati temporanei

-- Prima common table expression
WITH seniors AS (  
    SELECT student_id, first_name FROM STUDENTS.personal_info WHERE graduation_year = 2025

-- Seconda common table expression
), final_exam_grades AS (                           
    SELECT student_id, course_name, exam_score FROM STUDENTS.grades WHERE exam_type = 'Final'
)


SELECT
    seniors.first_name, final_exam_grades.course_name, final_exam_grades.exam_score
FROM final_exam_grades

-- Unisci i risultati temporanei
JOIN seniors ON final_exam_grades.student_id = seniors.student_id

Unire risultati temporanei

             first_name  |  course_name  |  exam_score
            ------------ | ------------- | ------------
              Ryan       |  Calculus I   |      97   
              Tatiana    |  Biology      |      98
              Pankaj     |  English III  |      92
              Taylor     |  Python       |      71
              Iris       |  Finance      |      89
              Charles    |  Marketing    |      88

                                ...

Usare un risultato temporaneo in una CTE

WITH ny_schools AS (
    SELECT school_id, school_name, district FROM SCHOOL.school_info WHERE school_state = 'NY'
), ny_teachers AS (
    SELECT
        teacher_id, teacher_name, tenure, specialty
    FROM SCHOOLS.teachers

    WHERE school_id IN (SELECT school_id FROM ny_schools)  -- Filtra per record in ny_schools

)

SELECT
    ny_teachers.teacher_name, ny_teachers.course_name, previous_courses.term
FROM SCHOOLS.previous_courses
JOIN ny_teachers ON previous_courses.teacher_id = ny_teachers.teacher_id AND
    previous_courses.course_area = ny_teachers.specialty
;

Usare un risultato temporaneo in una CTE

WITH ny_schools AS (
    SELECT 
        school_id, 
        ...
    WHERE school_state = 'NY'
), ny_teachers AS (
    SELECT
        ...
    FROM SCHOOLS.teachers


    -- Filtra per school_id nei record sopra
    WHERE school_id IN (
        SELECT school_id FROM ny_schools
    )

)

Prima filtra solo gli school_id di NY
Risultato temporaneo salvato in ny_schools
In ny_teachers, filtra per school_id presenti in ny_schools
Si può anche fare JOIN, ecc.

Allineare i carichi dei docenti alle loro specialità

...  -- Creazione della CTE ny_teachers

SELECT
    ny_teachers.teacher_name, 
    ny_teachers.course_name, 
    previous_courses.term
FROM SCHOOLS.previous_courses

-- Infine, unisci la CTE con la tabella previous_courses
JOIN ny_teachers ON 
    previous_courses.teacher_id = ny_teachers.teacher_id AND
    previous_courses.course_area = ny_teachers.specialty
;

Allineare i carichi dei docenti alle loro specialità

         teacher_name     |       course_name       |  term  
      ------------------- | ----------------------- | -------
       Marcus Lee         |  U.S. History           |  H1'22
       Priya Desai        |  Algebra II             |  H2'22
       Elena Rodríguez    |  Environmental Science  |  H1'25
       Jamal Thompson     |  English I              |  H2'21
       Mei-Ling Chen      |  World History          |  H1'23
       Gregory O'Malley   |  Calculus II            |  H2'24

                                ...

Passiamo alla pratica !

Tipi di dati e funzioni in Snowflake