select id,Count(id) as LoginT from A
    inner join B on
    id=aid
    group by id

if OBJECT_ID('ta') is not null Drop table ta;
if OBJECT_ID('tb') is not null Drop table tb;
go
create table ta(id int, name varchar(16));
create table tb(Aid int, LoginTime varchar(14));
go
insert into ta(id, name)
select 526, 'aaa' union all 
select 527, 'bbb' union all 
select 528, 'bbb';

insert into tb(Aid, LoginTime)
select 526, '20120404202903' union all 
select 526, '20120406202933' union all 
select 527, '20120406203006'

select id, sum(ct) as [count]
from (
        select ta.*, (case when aid is null then 0 else 1 end) as ct
        from ta
            left join tb 
            on id = aid
      ) a
group by id