一个选项使用 union all 以及 not exists :

select costc, description, budget 
from budget 
where costc in ('c1', 'c2', 'c3')
union all
select costc, description, budget 
from budget
where not exists (select 1 from budget where costsc in ('c1', 'c2', 'c3'))

也可以使用窗口功能：

select costc, description, budget 
from (
    select 
        b.*, 
        max(case when costc in ('c1', 'c2', 'c3') then 1 else 0 end) over() has_match
    from budget b
) b
where costc in ('c1', 'c2', 'c3') or has_match = 0

您可以从@gmb的答案重写union：

select costc, description, budget 
from budget 
where costc in ('c1', 'c2', 'c3')
or not exists (select 1 from budget where costc in ('c1', 'c2', 'c3'))

由此产生的执行计划可能与工会的执行计划相同。
或者，如果生成一个所有costc的列表比较便宜：

select something
from manymanyjoinsquery 
where costc in 
(  select costc 
        from budget 
        where costc in ('c1', 'c2', 'c3')
        or not exists (select 1 from budget where costsc in ('c1', 'c2', 'c3'))
)