如何将（未分组的）列添加到分组表中？

Question

I am a novice in SQL programming, who needs to solve this little exercise.

Exercise:

For each country, find the year, in which the maximal number of ships had been launched. In the case of more than one year in question, take a minimal year.

I have two tables, which are linked by the foreign key "class" (please see below):

CLASSES

SHIPS

What I need is the following result:

(maxNumber=count the number of years (column:"launched") for each country)

I have the following query which gives me - almost - the correct result:

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

RESULT:

My problem: How do I get the column "YEAR" in the table without adding it to the GROUP-function? If I just add "year" in the GROUP-function, the Maxnumber will be split in Years to. All I need is to add the relevant year to each line in the table (as shown in the required result).

I hope you SQL wizards can help me solving this.

thanks. Br, Jakob

Answer 1

This part is a little redundant:

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

You can just do this:

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

Then you can use ROW_NUMBER to assign the row with the highest count per country a 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

Then just do an outer select to get the rows where RN = 1 :

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