Pivot dei dati

Tipi di dati e funzioni in Snowflake

Jake Roach

Field Data Engineer

Tabella aggregata

                   course_name  |  exam_type  |  avg_exam_score
                  ------------- | ----------- | ----------------
                    Calculus I  |  Exam 1     |       81.78
                    Calculus I  |  Exam 2     |       83.55
                    Calculus I  |  Final      |       80.93

                    Finance     |  Exam 1     |       89.47
                    Finance     |  Exam 2     |       90.39
                    Finance     |  Final      |       89.69

                    Marketing   |  Exam 1     |       94.11
                    Marketing   |  Exam 2     |       93.29
                    Marketing   |  Final      |       93.81

Tabella “pivotata”




         course_name  |    "Exam 1"   |    "Exam 2"   |  "Final"
        ------------- | ------------- | ------------- | ---------
          Calculus I  |     81.78     |     83.55     |   80.93
          Finance     |     89.47     |     90.39     |   89.69
          Marketing   |     94.11     |     93.29     |   93.81

Creare una tabella pivotata

SELECT
    *
FROM SCHEMA.TABLE


PIVOT(

    -- Funzione di aggregazione
    SUM(<1>)


    -- Specifica le righe da trasformare in colonne
    FOR <2> IN (ANY ORDER BY <2>)

    -- Nessun GROUP BY!
);

PIVOT offre un modo diverso di mostrare dati aggregati “ruotando” valori in colonne

SELECT *, si può usare EXCLUDE
PIVOT va dopo FROM ...
Valori ANY in <2>

<1>: campo da aggregare

<2>: righe da trasformare in colonne

CTE e dati pivotati

WITH exam_grades AS (
    SELECT
        ..., exam_score, exam_type
    FROM SCHEMA.TABLE
    WHERE ...
)


SELECT
    *   -- Si può anche usare EXCLUDE
FROM exam_grades
PIVOT(
    AVG(exam_score)
    FOR exam_type IN (ANY ORDER BY exam_type)
);

Per prima cosa, definisci una CTE prima di usare PIVOT!

SELECT * FROM <cte>

exam_score: campo da aggregare

exam_type: righe da trasformare in colonne

Confrontare i voti per tipo d'esame

WITH exam_grades AS (
    SELECT 
        course_name, course_abbreviation, exam_score, exam_type
    FROM STUDENTS.grades
    WHERE course_level = '101'
)


SELECT
    * EXCLUDE course_abbreviation  -- Rimuovi course_abbrevation dal result set
FROM exam_grades

PIVOT(
    AVG(exam_score)
    FOR exam_type IN (ANY ORDER BY exam_type)
);

Confrontare i voti per tipo d'esame

                 course_name  |   exam_type   |  avg_exam_score
                ------------- | ------------- | ----------------
                  Calculus I  |   Exam 1      |       81.78
                  Calculus I  |   Exam 2      |       83.55
                  Calculus I  |   Final       |       80.93

                  Finance     |   Midterm 1   |       89.47
                                     ...

                 course_name  |    "Exam 1 "  |    "Exam 2"   |  "Final"
                ------------- | ------------- | ------------- | ---------
                  Calculus I  |     81.78     |     83.55     |   80.93
                  Finance     |     89.47     |     90.39     |   89.69
                  Marketing   |     94.11     |     93.29     |   93.81

Vamos praticar!

Tipi di dati e funzioni in Snowflake