Multiple common table expressions

Data Types and Functions in Snowflake

Jacob Roach

Field Data Engineer

Common table expressions (CTEs)

WITH seniors AS ( 

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

)


SELECT
    ...
FROM seniors

CTEs temporarily store the results of a query to eventually be used within another query

Used to organize queries
More readable and modular
Not limited to a single CTE

Defining multiple CTEs

Multiple temporary results can be defined using a single WITH statement

...), <another-cte-name> AS (...)
Make table filtering and manipulation easier to understand
JOIN multiple temporary result sets together
Subqueries within another CTE

 WITH <cte-name> AS (

     <query>

 ), <another-cte-name> AS (

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


 -- These CTE's could be JOIN'd 
 SELECT ... ;

Joining temporary result sets

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

-- Second 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

-- Join the temporary result sets together
JOIN seniors ON final_exam_grades.student_id = seniors.student_id

Joining temporary result sets

             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

                                ...

Using a temporary result in a 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)  -- Filter by records 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
;

Using a temporary result in a CTE

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


    -- Filter by school_id in records above
    WHERE school_id IN (
        SELECT school_id FROM ny_schools
    )

)

First, only filter for school_id's in NY
Temporary result stored in ny_schools
In ny_teachers, filter records by school_id in ny_schools
Could also JOIN, etc.

Matching teachers workloads to their specialties

...  -- Building the ny_teachers CTE

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

-- Finally, join the CTE with the previous_courses table
JOIN ny_teachers ON 
    previous_courses.teacher_id = ny_teachers.teacher_id AND
    previous_courses.course_area = ny_teachers.specialty
;

Matching teachers workloads to their specialties

         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

                                ...

Let's practice!

Data Types and Functions in Snowflake