MySQL组通过与顶级N数量的各类

Question

我有一个表这样的：

Rank      Letter
1         A
2         A
3         B
4         A
5         C
6         A
7         C
8         C
9         B
10        C 

我需要顶2的每个字母排序上升的等级：

Rank      Letter
1         A
2         A
3         B
5         C
7         C
9         B

我该怎么做?它相当直接得到的只是顶部使用1小组由，但我似乎无法得到它的工作的多个项

Answer 1

select distinct rank, letter
  from table1 t2
 where rank in 
         (select top 2 rank
            from table1 t2 
           where t2.letter = t1.letter 
           order by rank)
       order by letter, rank

编辑：（我第一次尝试不会MySql上（Quassnoi评论）工作，我修改了它例如SQL服务器上运行）

第二次尝试：

select t.letter, t.rank
from table1 t
join (
    select t1.letter, min(t1.rank) m
    from table1 t1
    join (select t0.letter, min(t0.rank) m, count(1) c 
           from table1 t0 group by t0.letter) t2
    on t1.letter = t2.letter and ((t2.c = 1) or (t2.c > 1 and t1.rank > m))
    group by t1.letter) t3 
  on t.letter = t3.letter and t.rank <= t3.m

Answer 2

SELECT  mo.Letter, md.Rank
FROM    (
        SELECT  DISTINCT letter
        FROM    mytable
        ) mo
JOIN    mytable md
ON      md.Letter >= mo.Letter
        AND md.Letter <= mo.Letter
        AND Rank <=
        COALESCE
                (
                (
                SELECT  Rank
                FROM    mytable mi
                WHERE   mi.letter = mo.letter
                ORDER BY
                        Rank
                LIMIT 1, 1
                ),
                0xFFFFFFFF
                )

你需要有一个综合指数上 (Letter, Rank) (按此顺序)

注意这个结构：

md.Letter >= mo.Letter
AND md.Letter <= mo.Letter

而不是仅仅 md.Letter = mo.Letter

它的力量 Range checked for each record 这是更有效率。

看看这篇文章在我的博客：

• 进行取样

更多详细信息。