每组SQL分组和限制结果

Question

我有一张桌子（你可以在这里看到http://sqlfiddle.com/#!9/009e8/1）和电影，每部电影都有一个类型和发布日期。

CREATE TABLE Movie (
    ID int,
    Title varchar(255),
    Genre varchar(255),
    Year int
);

INSERT into Movie VALUES(1, 'Movie 1', 'Horror', 1982);
INSERT into Movie VALUES(1, 'Movie 2', 'Drama', 1983);
INSERT into Movie VALUES(1, 'Movie 3', 'Horror', 1984);
INSERT into Movie VALUES(1, 'Movie 4', 'Comedy', 1985);
INSERT into Movie VALUES(1, 'Movie 5', 'Sci-Fi', 1986);
INSERT into Movie VALUES(1, 'Movie 6', 'Horror', 1987);
INSERT into Movie VALUES(1, 'Movie 7', 'Drama', 2010);
INSERT into Movie VALUES(1, 'Movie 8', 'Horror', 2008);
INSERT into Movie VALUES(1, 'Movie 9', 'Horror', 2006);
INSERT into Movie VALUES(1, 'Movie 10', 'Comedy', 2004);
INSERT into Movie VALUES(1, 'Movie 11', 'Horror', 2003);
INSERT into Movie VALUES(1, 'Movie 12', 'Drama', 2002);
INSERT into Movie VALUES(1, 'Movie 13', 'Comedy', 1999);
INSERT into Movie VALUES(1, 'Movie 14', 'Drama', 1967);
INSERT into Movie VALUES(1, 'Movie 15', 'Sci-Fi', 1982);
INSERT into Movie VALUES(1, 'Movie 16', 'Drama', 2009);
INSERT into Movie VALUES(1, 'Movie 17', 'Sci-Fi', 2008);

我希望能够获得具有特定类型（SELECT * FROM Movie WHERE Genre in ('Horror', 'Drama')）的所有电影，但限制按年份排序的每种类型的结果

所以，如果我有100部戏剧电影，100部恐怖片和100部喜剧，我只想要两首最古老的戏剧和两部最古老的恐怖电影。

如何在Sql中执行此操作？如果在标准SQL中这是不可能的，我正在使用Postgres

Answer 1

SELECT
  *
FROM (SELECT
  id,
  title,
  genre,
  year,
  DENSE_RANK() OVER (PARTITION BY genre ORDER BY year) AS year_calc
FROM Movie
WHERE Genre IN ('Horror', 'Drama')) a
WHERE year_calc IN (1, 2);

Saravanan

Answer 2

正如@Arijit Mukherjee建议你可以使用CTE和ROW_NUMBER()

;WITH CTE AS (
  SELECT *, seq = ROW_NUMBER() OVER (PARTITION BY Genre ORDER BY Year ASC)
  FROM Movie
  WHERE Genre in ('Horror', 'Drama')
)
SELECT * FROM CTE WHERE seq <= 2

Answer 3

SELECT rank_filter.* FROM (
    SELECT Movie.*, 
    rank() OVER (
        PARTITION BY Genre
        ORDER BY Year
    )
    FROM Movie
) rank_filter WHERE RANK IN (1,2)

http://sqlfiddle.com/#!17/009e8/13