use tempdb
;
create table rates (
    dt datetime primary key
    ,rate decimal(15,4))
    
;
create table flow (
    dt datetime primary key)
    
;
insert into rates(dt,rate)
select
    '20200701',10
union all select
    '20200703',15
union all select
    '20200705',20
;
insert into flow(dt)
select
    '20200701'
union all select
    '20200702'
union all select
    '20200703'
union all select
    '20200704'
union all select
    '20200705'
;
select
    f.dt flow_date
    ,r.dt rate_date
    ,r.rate rate_on_flow_date
from flow f
outer apply    (
    select top 1
        r.dt
        ,r.rate
    from rates r
    where
        r.dt<=f.dt
    order by
        r.dt desc) r
order by
    f.dt
;
drop table rates
;
drop table flow