SQL Server 2008の複数の列のCOUNT（DISTINCT）

cross applyを使用できます および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-データ

または、union allを実行することもできます サブクエリで。

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