Common Table Expressions

Snowflake में डेटा मैनिपुलेशन

Jake Roach

Field Data Engineer

Common Table Expressions

Common Table Expressions (CTE) एक क्वेरी के नतीजे अस्थायी रूप से सहेजती हैं ताकि दूसरी क्वेरी उन्हें इस्तेमाल कर सके

CTE क्वेरी की शुरुआत में परिभाषित होती है
WITH <cte-name> AS ( <query> )
सबक्वेरी जैसी

-- CTE का नाम दें,
-- और कोष्ठक में क्वेरी लिखें
WITH <cte-name> AS (
    <query>
)


SELECT
    ...
FROM <cte-name>
... ;

जोखिम वाले छात्रों पर रिपोर्टिंग

WITH at_risk AS (
    SELECT
        student_id
        course_name,
        teacher_name,
        grade
    FROM student_courses
    WHERE grade < 70 AND is_required
)


SELECT
    students.student_name,
    at_risk.*
FROM at_risk
JOIN students ON at_risk.student_id = students.id;

अस्थायी नतीजे at_risk में सहेजे गए
रिकॉर्ड्स का एक सबसेट चुनें
रिपोर्ट के लिए at_risk पर क्वेरी करें

जोखिम वाले छात्रों पर रिपोर्टिंग

CTE के नतीजे दिखाते हुए: सबसेट और अंतिम रिपोर्ट

Subqueries और CTE की तुलना

SELECT
    month_num,
    AVG(differential) AS avg_differential
    MIN(differential) AS most_differential
FROM (

    SELECT
        month_num,
        windchill - temperature AS differential
    FROM weather
    WHERE 
        season = 'Winter' AND
        temperature < 32

)
GROUP BY month_num;

WITH daily_temperature_differential AS (
    SELECT
        month_num,
        windchill - temperature AS differential
    FROM weather
    WHERE 
        season = 'Winter' AND 
        temperature < 32
)

SELECT
    month_num,
    AVG(differential) AS avg_differential
    MIN(differential) AS most_differential
FROM daily_temperature_differential
GROUP BY month_num;

तापमान अंतर ढूँढना

WITH daily_temperature_differential AS (
    SELECT
        month_num,
        windchill - temperature AS differential
    FROM weather
    WHERE 
        season = 'Winter' AND 
        temperature < 32
)


SELECT
    month_num,
    AVG(differential) AS avg_differential
    MIN(differential) AS most_differential
FROM daily_temperature_differential
GROUP BY month_num;

          | month_num | differential |
          | --------- | ------------ |
          |     12    |      -12     |
          |     1     |      -3      |
          |     1     |       0      |
          |     2     |      -7      |

यह प्रवाह subqueries से अधिक स्वाभाविक है.

| month_num | avg_differential | most_differential |
| --------- | ---------------- | ----------------- |
|     12    |       -5.77      |        -14        |
|     1     |       -1.91      |        -8         |
|     2     |       -8.13      |        -22        |

अभ्यास करते हैं!

Snowflake में डेटा मैनिपुलेशन