多個 Common Table Expressions

Snowflake 的資料型別與函式

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

CTE 會暫存查詢結果，以便在其他查詢中使用

用來整理查詢
更易讀、可模組化
不限於單一 CTE

定義多個 CTE

可在單一 WITH 敘述中定義多個暫時結果

...), <another-cte-name> AS (...)
讓表格篩選與處理更好懂
可將多個暫時結果 JOIN 起來
在另一個 CTE 中放入子查詢

 WITH <cte-name> AS (

     <query>

 ), <another-cte-name> AS (

     -- Add another query!
     <another-query>
 )


 -- 這些 CTE 可以一起 JOIN 
 SELECT ... ;

將暫時結果集合起來 JOIN

-- 第一個 CTE
WITH seniors AS (  
    SELECT student_id, first_name FROM STUDENTS.personal_info WHERE graduation_year = 2025

-- 第二個 CTE
), 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

-- 將暫時結果集合起來 JOIN
JOIN seniors ON final_exam_grades.student_id = seniors.student_id

將暫時結果集合起來 JOIN

             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

                                ...

在 CTE 中使用暫時結果

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)  -- 依 ny_schools 的紀錄篩選

)

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
;

在 CTE 中使用暫時結果

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


    -- 以上用 school_id 篩選
    WHERE school_id IN (
        SELECT school_id FROM ny_schools
    )

)

先只篩 NY 的 school_id
暫時結果存於 ny_schools
在 ny_teachers 中，依 ny_schools 的 school_id 篩選
也可改用 JOIN 等方式

將教師工作量對應其專長

...  -- 建立 ny_teachers 這個 CTE

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

-- 最後將 CTE 與 previous_courses 資料表 JOIN
JOIN ny_teachers ON 
    previous_courses.teacher_id = ny_teachers.teacher_id AND
    previous_courses.course_area = ny_teachers.specialty
;

將教師工作量對應其專長

         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

                                ...

一起來練習吧！

Snowflake 的資料型別與函式