统计单个查询中子表的所有条目

declare @Number int = number;


select  M.IdMain
        , SUM( Count(C1.Id) + Count(C2.Id) ) as TotalSum
from    Main M, Child1 C1, Child2 C2
where   IdMain = @Number
and     M.IdMain = C1.Id
and     M.IdMain = C2.Id
group 
by      M.IdMain

像这样的方法应该会奏效:

SELECT m.IdMain,
       COUNT(c1.id),
       COUNT(c2.id)
FROM MainTable m
LEFT OUTER JOIN Child1 c1
  ON c1.id = m.IdMain
LEFT OUTER JOIN Child2 c2
  ON c2.id = m.IdMain
WHERE m.IdMain = SomeNumber
GROUP BY m.IdMain;

您可以在两个子查询连接上使用完全外部连接,如下所示

See Working demo

create table MainTable (idMain int);
insert into MainTable values (1),(2),(3),(4)
create table Child1 (idChild int);
insert into Child1 values (1),(1),(2),(3),(3),(3)
create table Child2 (idChild int);
insert into Child2 values (2),(2),(2),(3)

select 
A.idMain,
ISNULL(countA,0) +ISNULL(countB,0) as count
from
(
    select 
    M.idMain,count(1) countA 
    from MainTable M 
    join Child1 A 
    on A.idChild=M.idMain 
    group by M.idMain
)A 
full outer join
(
    select 
        M.idMain,count(1) countB 
    from MainTable M 
    join Child2 B 
    on B.idChild=M.idMain 
    group by M.idMain
 )B
 on A.idMain=B.idMain
 --- your where clause goes here