count and sum the values sql

我有两张表：国家1，县2，国家边界，国家，年份和人口的边界，我想为每个国家列出其人口和所有邻国的总人口（如果它没有邻国，NULL）到目前为止，我可以找到每个国家的最大人口以及每个国家有多少邻居，但我无法找到总结所有邻国人口的方法。有任何想法吗？count and sum the values sql

SELECT country, MAX(population) FROM COUNTRYPOPS GROUP BY 1 ORDER BY 1 
--------------- 
SELECT countries, COUNT(countries) as have_borders 
FROM 
(SELECT country1 AS countries 
FROM borders 
UNION ALL 
SELECT country2 
FROM borders) t 
GROUP BY countries 
ORDER BY countries;

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2017-02-11 jack

这是MySql还是SQL Server？请相应标记。 –

是'BORDERS'双向？例如。一行'美国'，'加拿大'，另一行加'加拿大'，'美国'，还是它只有一行（任意）方向？ – Andreas

是的，这是双向的 – jack

如何像

SELECT country, pop, SUM(cc.population) AS neighboursPop 
FROM (
    SELECT c.country AS country, c.population AS pop, b.country2 AS neighbour 
    FROM countrypops c 
     LEFT JOIN borders b ON b.country1 = c.country 
    ) t 
     LEFT JOIN countrypops cc ON cc.country = t.neighbour 
GROUP BY country;

没有测试它，但这个想法是。

UPD：或者它甚至可以被夷为平地像

SELECT country, pop, SUM(neighPop) AS neighboursPop 
FROM (
    SELECT 
     c.country AS country, 
     c.population AS pop, 
     b.country2 AS neighbour, 
     cc.population AS neighPop 
    FROM countrypops c 
     LEFT JOIN (borders b 
        LEFT JOIN countrypops cc ON cc.country = b.country2) 
     ON b.country1 = c.country 
    ) t 
GROUP BY country;

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2017-02-11 01:52:47

我认为相关子查询是最简单的方法：

select cp.country, cp.pop, 
     (select sum(cp2.pop) 
     from borders b join 
      countrypops cp2 
      on b.country2 = cp2.country 
     where b.country1 = cp.country 
     ) as neighboring_pop 
from countrypops cp;

这并不一定在外部查询的聚集，所以它应该比一种方法更有效率。

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2017-02-11 03:19:42

count and sum the values sql

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