在GROUP BY中使用LIMIT得到每一個組N個結果？

如下查詢：

SELECT
year, id, rate
FROM h
WHERE year BETWEEN 2000 AND 2009
AND id IN (SELECT rid FROM table2)
GROUP BY id, year
ORDER BY id, rate DESC

產量：

year    id  rate
2006    p01 8
2003    p01 7.4
2008    p01 6.8
2001    p01 5.9
2007    p01 5.3
2009    p01 4.4
2002    p01 3.9
2004    p01 3.5
2005    p01 2.1
2000    p01 0.8
2001    p02 12.5
2004    p02 12.4
2002    p02 12.2
2003    p02 10.3
2000    p02 8.7
2006    p02 4.6
2007    p02 3.3

我想要的只是每一個ID的前5個結果：

2006    p01 8
2003    p01 7.4
2008    p01 6.8
2001    p01 5.9
2007    p01 5.3
2001    p02 12.5
2004    p02 12.4
2002    p02 12.2
2003    p02 10.3
2000    p02 8.7

有沒有一種方法能夠使用在GROUP BY中起做用的某種LIMIT之類的修飾符？

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#1樓

如下文章： sql：選擇每一個組的前N條記錄描述了無需子查詢便可實現此目的的複雜方法。

它對如下提供的其餘解決方案進行了改進：

• 在一個查詢中完成全部操做
• 可以正確利用索引
• 避免衆所周知的子查詢會在MySQL中產生錯誤的執行計劃

可是，它並不漂亮。 若是在MySQL中啓用了窗口函數（又稱分析函數），則能夠實現一個好的解決方案，但事實並不是如此。 該文章中使用的技巧利用了GROUP_CONCAT，有時將其稱爲「 MySQL的窮人窗口函數」。

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#2樓

這須要一系列子查詢來對值進行排序，限制它們，而後在分組時執行求和

@Rnk:=0;
@N:=2;
select
  c.id,
  sum(c.val)
from (
select
  b.id,
  b.bal
from (
select   
  if(@last_id=id,@Rnk+1,1) as Rnk,
  a.id,
  a.val,
  @last_id=id,
from (   
select 
  id,
  val 
from list
order by id,val desc) as a) as b
where b.rnk < @N) as c
group by c.id;

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#3樓

SELECT year, id, rate
FROM (SELECT
  year, id, rate, row_number() over (partition by id order by rate DESC)
  FROM h
  WHERE year BETWEEN 2000 AND 2009
  AND id IN (SELECT rid FROM table2)
  GROUP BY id, year
  ORDER BY id, rate DESC) as subquery
WHERE row_number <= 5

子查詢與您的查詢幾乎相同。 惟一的變化是增長

row_number() over (partition by id order by rate DESC)

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#4樓

對於那些像我這樣查詢超時的人。 我作了如下內容以使用限制和特定組的其餘任何內容。

DELIMITER $$
CREATE PROCEDURE count_limit200()
BEGIN
    DECLARE a INT Default 0;
    DECLARE stop_loop INT Default 0;
    DECLARE domain_val VARCHAR(250);
    DECLARE domain_list CURSOR FOR SELECT DISTINCT domain FROM db.one;

    OPEN domain_list;

    SELECT COUNT(DISTINCT(domain)) INTO stop_loop 
    FROM db.one;
    -- BEGIN LOOP
    loop_thru_domains: LOOP
        FETCH domain_list INTO domain_val;
        SET a=a+1;

        INSERT INTO db.two(book,artist,title,title_count,last_updated) 
        SELECT * FROM 
        (
            SELECT book,artist,title,COUNT(ObjectKey) AS titleCount, NOW() 
            FROM db.one 
            WHERE book = domain_val
            GROUP BY artist,title
            ORDER BY book,titleCount DESC
            LIMIT 200
        ) a ON DUPLICATE KEY UPDATE title_count = titleCount, last_updated = NOW();

        IF a = stop_loop THEN
            LEAVE loop_thru_domain;
        END IF;
    END LOOP loop_thru_domain;
END $$

它遍歷一個域列表，而後每一個域只插入200個限制

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#5樓

嘗試這個：

SELECT h.year, h.id, h.rate 
FROM (SELECT h.year, h.id, h.rate, IF(@lastid = (@lastid:=h.id), @index:=@index+1, @index:=0) indx 
      FROM (SELECT h.year, h.id, h.rate 
            FROM h
            WHERE h.year BETWEEN 2000 AND 2009 AND id IN (SELECT rid FROM table2)
            GROUP BY id, h.year
            ORDER BY id, rate DESC
            ) h, (SELECT @lastid:='', @index:=0) AS a
    ) h 
WHERE h.indx <= 5;