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：（我的第一次尝试将无法在MysqL（Quassnoi注释）上工作，例如，我将其修改为可在sql服务器上工作）
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
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