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  • MySQL笔记9

    1.Left and Right joins

    '''
    A right outer join will include all of the rows of the table to the right of the RIGHT JOIN clause.
    '''
    %%sql
    SELECT r.dog_guid AS rDogID, d.dog_guid AS dDogID, r.user_guid AS rUserID, d.user_guid AS dUserID, AVG(r.rating) AS AvgRating, COUNT(r.rating) AS NumRatings, d.breed, d.breed_group, d.breed_type
    FROM dogs d RIGHT JOIN reviews r
      ON r.dog_guid=d.dog_guid AND r.user_guid=d.user_guid
    WHERE r.dog_guid IS NOT NULL
    GROUP BY r.dog_guid
    HAVING NumRatings >= 10
    ORDER BY AvgRating DESC
    LIMIT 10;

    Example2

    %%sql 
    SELECT DISTINCT u.user_guid AS UserID, COUNT(d.dog_guid) AS Numberdogs
    FROM users u LEFT JOIN dogs d
    ON u.user_guid = d.user_guid
    GROUP BY UserID 
    ORDER BY Numberdogs DESC
    LIMIT 10;

    Example3:

    '''
    Question 10: How would you write a query that used a left join to return the number of distinct user_guids that were in the users table, but not the dogs table (your query should return a value of 2226)?
    '''
    %%sql 
    SELECT  COUNT(DISTINCT u.user_guid) AS Number
    FROM  users u LEFT JOIN  dogs d
    ON u.user_guid = d.user_guid
    WHERE d.user_guid IS NULL;

    2.关于 COUNT DISTINCT 的注意点。

    '''
    That's because COUNT DISTINCT does NOT count NULL values, 
    while SELECT/GROUP BY clauses roll up NULL values into one group.
    If you want to infer the number of distinct entries from the results of a query using joins and GROUP BY clauses,
    remember to include an "IS NOT NULL" clause to ensure you are not counting NULL values '''
    The Safest Way to Get what you Want is to Try and Deserve What you Want.
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  • 原文地址:https://www.cnblogs.com/Shinered/p/9614448.html
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