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Finding Out Country's Population By Using Self Joins
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    Comparing a country to itself Self joins are very useful for comparing data from one part of a table with another part of the same table. Here, I am interested in finding out how much the populations for each country changed from 2010 to 2015, and will query this change by performing a self join.

    In this exercise, we'll work to answer this question by joining the populations table with itself.

    Unknown integration
    Queryavailable as
        p1.size AS size2010, 
        p2.size AS size2015
    FROM world.populations AS p1
    INNER JOIN world.populations AS p2
    ON p1.country_code = p2.country_code
    WHERE p1.year = 2010 AND
    -- Filter such that p1.year is always five years before p2.year
    p2.year = 2015;