如何将（未分组的）列添加到分组表中？

Question

我是SQL编程的新手，需要解决此小问题。

行使：

对于每个国家/地区，请找出发行最大船舶数量的年份。 如果问题一年以上，请花最少的时间。

我有两个表，这些表通过外键“类”链接（请参见下文）：

类

船舶

我需要的是以下结果：

（maxNumber =计算每个国家/地区的年限（列：“启动”））

我有以下查询，几乎可以给我正确的结果：

SELECT e.country, MAX(e.number) as maxNumber
FROM(
    SELECT d.country, d.Year, count(*) as number
    FROM(
        SELECT a.country, b.launched AS Year
        FROM Classes a 
        INNER JOIN ships b ON a.class=b.class
        ) d
GROUP BY d.country, d.Year) e
GROUP BY e.country

结果：

我的问题：如何在表中获取“ YEAR”列而不将其添加到GROUP函数中？ 如果我只是在GROUP函数中添加“ year”，则Maxnumber将以Years为单位。 我需要做的就是将相关年份添加到表的每一行中（如所需结果所示）。

我希望您的SQL向导可以帮助我解决这个问题。

谢谢。 Br，雅各布

Answer 1

这部分有点多余：

SELECT d.country, d.Year, count(*) as number
FROM(
     SELECT a.country, b.launched AS Year
     FROM Classes a 
     INNER JOIN ships b ON a.class=b.class
     ) d
GROUP BY d.country, d.Year

您可以这样做：

SELECT a.country, b.launched AS Year, count(*) as Number
FROM Classes a 
INNER JOIN ships b ON a.class=b.class
GROUP BY a.country, b.launched

然后，您可以使用ROW_NUMBER为每个国家/地区中计数最高的行分配1 ：

SELECT a.country, b.launched AS Year, count(*) as Number,
       ROW_NUMBER() over (PARTITION BY a.country ORDER BY COUNT(*) DESC) as RN
FROM Classes a 
INNER JOIN ships b ON a.class=b.class
GROUP BY a.country, b.launched

然后只需执行外部选择即可获得RN = 1的行：

SELECT country, year, number
FROM (SELECT a.country, b.launched AS Year, count(*) as Number,
             ROW_NUMBER() over (PARTITION BY a.country ORDER BY COUNT(*) DESC) as RN
      FROM Classes a 
      INNER JOIN ships b ON a.class=b.class
      GROUP BY a.country, b.launched
     ) A
WHERE RN = 1