高级公用表表达式

Snowflake 中的数据操作

Jake Roach

Field Data Engineer

定义多个公用表表达式

可在单个 WITH 子句中定义多个公用表表达式（CTE）

可定义任意数量的 CTE
CTE 之间可进行 JOIN
仍可在 CTE 中执行复杂操作
甚至可在一个 CTE 中使用另一个的结果

WITH <cte-name> AS (

    <query>

), <another-cte-name> (

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

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

高表现课程

WITH active_courses AS (
    SELECT
        id,
        course_name,
        teacher_name
    FROM courses
    WHERE is_active

), course_avgs (
    SELECT
        course_id,
        AVG(grade) AS avg_grade
    FROM student_courses
    GROUP BY course_id
)


SELECT
    a.course_name,
    a.teacher_name,
    c.avg_grade
FROM active_courses AS a

-- JOIN these CTEs together
JOIN course_avgs AS c
    ON a.id = c.course_id

ORDER BY avg_grade DESC;

查询更易读！

高表现课程

将临时结果集合并为每门课程的平均分和教师姓名的最终输出

资深教师

WITH active_courses AS (
    SELECT
        id,
        course_name,
        teacher_name,
        teacher_tenure
    FROM courses

    -- JOIN the teachers table to the courses 
    -- table to get teacher_tenure
    JOIN teachers 
        ON courses.teacher_id = teachers.id

    WHERE is_active
), 
...

...
), course_avgs (
    SELECT
        course_id,
        AVG(grade) AS avg_grade
    FROM student_courses
    GROUP BY course_id
)


SELECT
    a.teacher_name,
    a.teacher_tenure
    MAX(c.avg_grade) AS highest_grade
FROM active_courses AS a
JOIN course_avgs AS c ON a.id = c.course_id
GROUP BY a.teacher_name, a.teacher_tenure;

资深教师

查询教师任期及其最高课程平均分的结果集

Passons à la pratique !

Snowflake 中的数据操作