create table A(a int, b int)
insert into a values(1 ,1)
insert into a values(2 ,2)
insert into a values(3 ,3)
insert into a values(4 ,4)
insert into a values(5 ,5)
create table b(a int, b int)
insert into b values(11 ,1)
insert into b values(22 ,2)
insert into b values(33 ,3)
insert into b values(4 ,4)
insert into b values(55 ,5)
create table C(a int, b int)
insert into c values(11 ,1)
insert into c values(2 ,2)
insert into c values(3 ,3)
insert into c values(44, 4)
insert into c values(55, 5)
go

select a , count(1) [sum] from 
(
select a , b from a
union all
select a , b from b
union all
select a , b from c
) t
group by a
 
/*
a           sum         
----------- ----------- 
1           1
2           2
3           2
4           2
5           1
11          2
22          1
33          1
44          1
55          2

（所影响的行数为 10 行）
*/

drop table a,b , c