获取SQL中另一列的每个值的最常见值

我有这样一张桌子:

 Column  | Type | Modifiers 
---------+------+-----------
 country | text | 
 food_id | int  | 
 eaten   | date | 

对于每个国家,我想获得最常吃的食物.我能想到的最好的(我使用的是postgres)是:

CREATE TEMP TABLE counts AS 
   SELECT country, food_id, count(*) as count FROM munch GROUP BY country, food_id;

CREATE TEMP TABLE max_counts AS 
   SELECT country, max(count) as max_count FROM counts GROUP BY country;

SELECT country, max(food_id) FROM counts 
   WHERE (country, count) IN (SELECT * from max_counts) GROUP BY country;

在最后一个陈述中,需要GROUP BY和max()来打破关系,其中两种不同的食物具有相同的数量.

对于概念上简单的事情来说,这似乎是很多工作.有没有更直接的方式来做到这一点？

在问到这个问题的那一年后,PostgreSQL 在8.4中引入了对窗口函数的支持.值得注意的是,今天可能会解决如下问题:

SELECT country, food_id
  FROM (SELECT country, food_id, ROW_NUMBER() OVER (PARTITION BY country ORDER BY freq DESC) AS rn
          FROM (  SELECT country, food_id, COUNT('x') AS freq
                    FROM country_foods
                GROUP BY 1, 2) food_freq) ranked_food_req
 WHERE rn = 1;

以上将打破关系.如果您不想断开关系,可以改用DENSE_RANK().

它现在更简单:PostgreSQL 9.4引入了这个mode()功能:

select mode() within group (order by food_id)
from munch
group by country

返回(如user2247323的示例):

country | mode
--------------
GB      | 3
US      | 1

请参阅此处的文档:https: //wiki.postgresql.org/wiki/Aggregate_Mode

https://www.postgresql.org/docs/current/static/functions-aggregate.html#FUNCTIONS-ORDEREDSET-TABLE

SELECT DISTINCT
"F1"."food",
"F1"."country"
FROM "foo" "F1"
WHERE
"F1"."food" =
    (SELECT "food" FROM
        (
            SELECT "food", COUNT(*) AS "count"
            FROM "foo" "F2" 
            WHERE "F2"."country" = "F1"."country" 
            GROUP BY "F2"."food" 
            ORDER BY "count" DESC
        ) AS "F5"
        LIMIT 1
    )

好吧,我写的很匆忙,并没有检查得很好.子选择可能非常慢,但这是我能想到的最短且最简单的SQL语句.当我喝醉的时候,我可能会告诉你更多.

PS:哦,好吧,"foo"是我桌子的名字,"food"包含食物的名称,"country"包含国家名称.样本输出:

   food    |  country   
-----------+------------
 Bratwurst | Germany
 Fisch     | Frankreich

试试这个:

Select Country, Food_id
From Munch T1
Where Food_id= 
    (Select Food_id
     from Munch T2
     where T1.Country= T2.Country
     group by Food_id
     order by count(Food_id) desc
      limit 1)
group by Country, Food_id