select a1,sum(a2) from
(select * from db1..table1
union all
select * from db2..table1
....
select * from dbN..table1)AA
group by a2
(select * from db1..table1
union all
select * from db2..table1
....
select * from dbN..table1)AA
group by a2
from
(select a1 ,a2 from db1.dbo.table1
union
select a1 ,a2 from db2.dbo.table1
union
select a1 ,a2 from db3.dbo.table1
) as T
group by a1
select * from db1..table1
union all
select * from db2..table1
....
select * from dbN..table1
select * from db1..table1
union all
select * from db2..table1
....
select * from dbN..table1