透视数据

Snowflake 中的数据类型与函数

Jake Roach

Field Data Engineer

汇总表

                   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

"透视"的表




         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

创建透视表

SELECT
    *
FROM SCHEMA.TABLE


PIVOT(

    -- 聚合函数
    SUM(<1>)


    -- 指定要从行转为列的字段
    FOR <2> IN (ANY ORDER BY <2>)

    -- 不需要 GROUP BY！
);

PIVOT 通过将值"透视"为列来输出聚合数据的另一种方式

SELECT * 可配合 EXCLUDE
PIVOT 写在 FROM ... 之后
<2> 中可用 ANY 值

<1>：要聚合的字段

<2>：要转成列的行值

CTE 与透视数据

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


SELECT
    *   -- 也可用 EXCLUDE
FROM exam_grades
PIVOT(
    AVG(exam_score)
    FOR exam_type IN (ANY ORDER BY exam_type)
);

先定义一个 CTE，再用 PIVOT！

        `SELECT * FROM <cte>`

exam_score：要聚合的字段

exam_type：要转成列的行值

按考试类型比较成绩

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


SELECT
    * EXCLUDE course_abbreviation  -- 从结果中移除 course_abbrevation
FROM exam_grades

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

按考试类型比较成绩

                 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!

Snowflake 中的数据类型与函数