SQL Server 2017

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create table lefttable (value int, program varchar(10), race int);
create table righttable (commission int, program varchar(10), race int);

insert into lefttable values
(100, 'gold', 1), (100, 'gold', 4), (100, 'gold', 5);

insert into righttable values
(14, 'gold', 1), (23, 'gold', null);
5 rows affected

select
    *
from 
    righttable r 
where 
    (r.race=1 and r.program = 'gold')
    or
    r.race is null
order by
    case when r.race is null then 1 else 0 end


commission
program
race


14
gold
1

23
gold
null


select
    l.race, 
    totalvalue = sum(value), 
    commission = SUM(value * (commission * 0.01))
from 
    lefttable l
outer apply
    (select top 1 
        commission 
     from 
         righttable r 
     where 
         (r.race=l.race and r.program = l.program)
         or
         r.race is null
     order by
         case when r.race is null then 1 else 0 end) r
group by
    l.race


race
totalvalue
commission


1
100
14.00

4
100
23.00

5
100
23.00