MySQL分组方式,每种类型前N个
我有一张这样的桌子: Rank Letter 1 A 2 A 3 B 4 A 5 C 6 A 7 C 8 C 9 B 10 C 我需要按升序排列每个字母的前2位: Rank Letter 1 A 2 A 3 B 5 C 7 C 9 B 我该怎么做?使用GROUPBY只获得前1名是相当简单的,但我似乎无法让它用于多个条目MySQL分组方式,每种类型前N个,mysql,group-by,ranking,Mysql,Group By,Ranking,我有一张这样的桌子: Rank Letter 1 A 2 A 3 B 4 A 5 C 6 A 7 C 8 C 9 B 10 C 我需要按升序排列每个字母的前2位: Rank Letter 1 A 2 A 3 B 5 C 7 C 9
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
)
而不仅仅是md.字母=mo.字母
它强制检查每个记录的范围,这样更有效
请参见我的博客中的这篇文章:
有关这方面的更多详细信息
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:我的第一次尝试在MySql上不起作用。例如,我修改了它以在sql server上工作
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
我得到一个错误1242-子查询返回超过1行,因为最后一个子查询的限制是2,你知道为什么吗?这是一个旧答案,但我发现它很有用。然而,我确实发现它并不像预期的那样有效。例如,如果一封信的实例少于2个,它将不会返回任何实例。您可以添加如下内容:或者从mytable mi中选择COUNT*,其中mi.letter=mo.letter@Quassnoi非常好-我更喜欢您的版本,谢谢:对不起,我更多地使用sql server。现在我必须为mysql找到另一个不同于您的解决方案;
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