在使用另一个GROUP BY时选择具有MAX值的行

Question

我是学习SQL的新手，所以我事先道歉，我的问题很基本。

我想要做的是显示一列，该列将告诉我在已区分的“ old_country”和“ young_country”组中，percent_aged_over_60值最高和最低的国家：

SELECT
    CASE
        WHEN percent_aged_over_60 >= 10 THEN "old_country"
        WHEN percent_aged_over_60 < 10 THEN "young_country"
        ELSE "error"
    END AS "oldness",
COUNT(*)
FROM countries_by_population
    GROUP BY oldness;

换句话说，我要添加一列，以告诉我该国家的名称，其中“ old_country”为MAX（percent_aged_over_60），而“ young_country”则为MIN（percent_aged_over_60）。

虽然我确信这是最简单的两个独立查询，但我想在同一查询中执行。

因此，我正在寻找一个产生三列的结果-年龄，计数（*）和国家。

Answer 1

使用此查询：

select t.country,t.oldness,count(*) from
(SELECT
 CASE
        WHEN percent_aged_over_60 >= 10 THEN "old_country"
        WHEN percent_aged_over_60 < 10 THEN "young_country"
        ELSE "error"
 END AS oldness
 , country
  ,*
 FROM countries_by_population) as t
 GROUP BY t.oldness,t.country;

Answer 2

使用子查询来获取最小和最大百分比，然后将其与表结合起来以获取匹配它们的国家/地区。

SELECT country,
        CASE
            WHEN percent_aged_over_60 >= 10 THEN "old_country"
            ELSE "young_country"
        END AS oldness,
        percent_aged_over_60
FROM countries_by_population
JOIN (SELECT MAX(percent_aged_over_60) AS max_pct, MIN(percent_aged_over_60) AS min_pct
      FROM countries_by_population) AS x
ON percent_aged_over_60 >= 10 AND percent_aged_over_60 = max_pct
    OR percent_aged_over_60 < 10 AND percent_aged_over_60 = min_pct

演示