select a.fno,a.ct1,b.ct2 from 
(
    select F.fno,isnull(count(B.bno),0) as ct1
    from forum F 
    left join board B 
    on F.fno = B.fno 
    group by F.fno
) a,
(
    select F.fno,isnull(count(U.uno),0) as ct2
    from forum F 
    left join [user] U 
    on F.fno = U.fno 
    group by F.fno    
) b
where a.fno=b.fno

------解决方案--------------------

SQL code

--> 测试数据: @forum
declare @forum table (fno int)
insert into @forum
select 1 union all
select 2 union all
select 3 union all
select 4

--> 测试数据: @board
declare @board table (bno int,fno int)
insert into @board
select 1,1 union all
select 2,2 union all
select 3,3 union all
select 4,4 union all
select 5,2

--> 测试数据: @user
declare @user table (uno int,fno int)
insert into @user
select 1,1 union all
select 2,2 union all
select 3,3 union all
select 4,4 union all
select 5,1

select * ,
(select count(1) from @board where fno=a.fno) as ct1 ,
(select count(1) from @user where fno=a.fno) as ct2
from @forum a
/*
fno         ct1         ct2
----------- ----------- -----------
1           1           2
2           2           1
3           1           1
4           1           1
*/

------解决方案--------------------
SQL code

select F.fno,F.name,F.url,F.pr,F.br,B.cou,U.cou,F.remark
from forum F
left join (select fno,count(1) cou from board group by fno) B on F.fno = B.fno
left join (select fno,count(1) cou from [user] group by fno) U on F.fno = U.fno