如何爲每個GROUP BY條件返回多個行

Question

我試圖獲得給定GROUP BY條件的TOP X結果。 我目前使用的是這樣的：

SELECT * FROM 
     (SELECT id 
      FROM myTable 
      WHERE id IN (x1, x2, ..., xn) GROUP BY id ORDER BY grade DESC 
     ) t1 
     INNER JOIN myTable t2 ON t2.id=t1.id 

ID是一個非唯一索引INT場，每個值多行。

這會爲每個ID返回最佳等級的行。我怎樣才能轉換這個返回每個ID的TOP X結果？

例如，從TOP X以下數據

id grade 
2 10 
2 13 
2 15 
3 20 
4 16 
4 55 
4 45 
4 35 
4 25 
5 1 

假設X是2，我想獲得的行：

id grade 
2 15 
2 13 
3 20 
4 50 
4 40 
5 1 

Answer 1

也許，這樣的事情？

SELECT m.* 
FROM (
    SELECT id 
    FROM myTable 
    WHERE id IN (1, 3) 
    GROUP BY id 
) AS ids 
RIGHT JOIN myTable AS m ON ids.id = m.id 
WHERE 
    m.id = ids.id AND 
    m.grade IN (
    SELECT TOP 5 t.grade 
    FROM myTable AS t 
    WHERE t.id = ids.id 
    ORDER BY t.grade DESC 
); 

UPD： 或者，甚至

SELECT m.* 
FROM myTable AS m 
WHERE 
    m.id IN (1, 2) AND 
    m.grade IN (
    SELECT TOP 5 t.grade 
    FROM myTable AS t 
    WHERE t.id = m.id 
    ORDER BY t.grade DESC 
); 

Answer 2

這取決於你是否不想被返回的關係，還是不行。

如果你想聯繫退回，您可以使用下面的方法：

CREATE TABLE grades (
    id INT, 
    grade INT 
); 

INSERT INTO grades VALUES (1, 2); 
INSERT INTO grades VALUES (1, 3); 
INSERT INTO grades VALUES (1, 4); 
INSERT INTO grades VALUES (1, 5); 

INSERT INTO grades VALUES (2, 5); 
INSERT INTO grades VALUES (2, 5); 
INSERT INTO grades VALUES (2, 5); 
INSERT INTO grades VALUES (2, 4); 

INSERT INTO grades VALUES (3, 3); 
INSERT INTO grades VALUES (3, 4); 

INSERT INTO grades VALUES (4, 3); 

SELECT id, grade 
    FROM grades g 
WHERE (
    SELECT COUNT(DISTINCT grade) FROM grades 
    WHERE id = g.id 
     AND grade >= g.grade 
) <= 2; 

輸出：

ID  GRADE 
1 4 
1 5 
2 5 
2 5 
2 5 
2 4 
3 3 
3 4 
4 3

如果您不希望聯繫，使用DISTINCT：

SELECT DISTINCT id, grade 
    FROM grades g 
WHERE (
    SELECT COUNT(DISTINCT grade) FROM grades 
    WHERE id = g.id 
     AND grade >= g.grade 
) <= 2; 

輸出：

ID  GRADE 
1 4 
1 5 
2 5 
2 4 
3 3 
3 4 
4 3

SQLFiddle：SQLFiddle