多个公用表表达式（CTE）

Snowflake 中的数据类型与函数

Jacob Roach

Field Data Engineer

公用表表达式（CTE）

WITH seniors AS ( 

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

)


SELECT
    ...
FROM seniors

CTE 会临时存储查询结果，以便在另一查询中使用

用于组织查询
更易读、模块化
不限于单个 CTE

定义多个 CTE

可在一个 WITH 语句中定义多个临时结果

...), <another-cte-name> AS (...)
使表筛选与操作更易理解
JOIN 多个临时结果集
在另一 CTE 中使用子查询

 WITH <cte-name> AS (

     <query>

 ), <another-cte-name> AS (

     -- 添加另一条查询！
     <another-query>
 )


 -- 这些 CTE 可进行 JOIN 
 SELECT ... ;

连接临时结果集

-- 第一个公用表表达式
WITH seniors AS (  
    SELECT student_id, first_name FROM STUDENTS.personal_info WHERE graduation_year = 2025

-- 第二个公用表表达式
), 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

-- 将临时结果集连接在一起
JOIN seniors ON final_exam_grades.student_id = seniors.student_id

连接临时结果集

             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 中使用临时结果

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)  -- 按 ny_schools 中的记录筛选

)

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 中使用临时结果

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


    -- 按上方记录中的 school_id 筛选
    WHERE school_id IN (
        SELECT school_id FROM ny_schools
    )

)

先仅筛选 NY 的 school_id
临时结果存入 ny_schools
在 ny_teachers 中按 ny_schools 的 school_id 筛选
也可用 JOIN 等方式

将教师工作与其专长匹配

...  -- 构建 ny_teachers CTE

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

-- 最后，将 CTE 与 previous_courses 表连接
JOIN ny_teachers ON 
    previous_courses.teacher_id = ny_teachers.teacher_id AND
    previous_courses.course_area = ny_teachers.specialty
;

将教师工作与其专长匹配

         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

                                ...

开始练习吧！

Snowflake 中的数据类型与函数