SQL - 更多记录之间的差异和平均值

Question

我有以下表COUNTRY_PEOPLE：

COUNTRY - CITY - YEAR - PEOPLE

ENGLAND - LONDON 1980 - 7834020
ENGLAND - LONDON 2010 - 8308369
ENGLAND - DERBY  1980 - 231483
ENGLAND - DERBY  2010 - 233700
FRANCE  - PARIS  1980 - 2174654
FRANCE  - PARIS  2010 - 2274880
FRANCE  - NANTES 1980 - 279321
FRANCE  - NANTES 2010 - 290130

我需要一个SQL查询来获取每个国家/地区的城市列表，其中包含2010年和1980年之间的人员差异。 所以：

ENGLAND - LONDON  474349
ENGLAND - DERBY   2217
FRANCE  - PARIS   100226
FRANCE  - NANTES  10809

然后是国家的平均值，但这应该是一组很容易的。 我刚刚从一个真实无聊的场景中发明了这个例子，所有数据都是假的。 提前谢谢

Answer 1

尝试：

 SELECT Country, City, 
  AVG(CASE WHEN Year=2010 THEN People END)-AVG(CASE WHEN Year=1980 THEN People END) as AvgPeople
 FROM COUNTRY_PEOPLE
 GROUP BY Country, City

参见 Demo

<强>结果：

Answer 2

请检查以下查询

SELECT a.COUNTRY,a.CITY,(ISNULL(a.PEOPLE,0) - ISNULL(b.PEOPLE,0)) AS PEOPLE
FROM
 (SELECT COUNTRY ,CITY  , SUM(ISNULL(PEOPLE,0)) AS PEOPLE FROM  COUNTRY_PEOPLE
   WHERE [YEAR] = 2010  GROUP BY COUNTRY ,CITY) a 
LEFT OUTER JOIN
(SELECT COUNTRY ,CITY  , SUM(ISNULL(PEOPLE,0)) AS PEOPLE FROM  COUNTRY_PEOPLE 
   WHERE [YEAR] = 1980 GROUP BY COUNTRY ,CITY)
    b ON a.COUNTRY = b.COUNTRY AND a.CITY = b.CITY

Answer 3

怎么样：

select c1.Country,c1.City,(c1.People-c2.People) as Diff from Country_people c1
join Country_people c2 on c1.Country = c2.Country and c1.City = c2.City and c1.Year = 2010 `and c2.Year = 1980`