Você pode usar uma
cross apply e values . select T1.AssetId,
count(distinct T2.ID) TotalIds,
sum(case T2.InStock when 0 then 0 else 1 end) AvailableIds
from YourTable as T1
cross apply(values(T1.MyId, T1.InStock),
(T1.TheirId, 0)
) as T2(ID, InStock)
group by T1.AssetId
SE-Data
Ou você pode fazer uma
union all em uma subconsulta. select T.AssetId,
count(distinct T.ID) TotalIds,
sum(case T.InStock when 0 then 0 else 1 end) AvailableIds
from (
select AssetId, MyId as ID, InStock
from YourTable
union all
select AssetID, TheirId, 0
from YourTable
) as T
group by T.AssetId
